boards: sabre-6quad: Add knsh

Summary:

- Add sabre-6quad:knsh to test BUILD_KERNEL with qemu
- Update README.txt to run the nuttx with qemu

Impact:
- None

Testing:
- Tested with QEMU
- NOTE: getprime crashes with undefined instruction when
  compiled with gcc 10.3.x (please use 9.3.1)

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

boards/arm/imx6/sabre-6quad/README.txt

@@ -903,3 +903,40 @@ Configuration sub-directories
     2. See the STATUS and SMP sections above for detailed SMP-related
        issues.  There are a some major problems with the current SMP
        implementation.
+
+  knsh
+  ---
+    This is a configuration of testing the BUILD_KERNEL configuration.
+
+    $ cd nuttx
+    $ ./tools/configure.sh sabre-6quad:knsh
+    $ make V=1 -j7
+    $ make export V=1
+    $ cd ../apps
+    $ ./tools/mkimport.sh -x ../nuttx/nuttx-export-*.zip
+    $ make import V=1
+    $ cd ../nuttx
+    $ qemu-system-arm -semihosting -M sabrelite -m 1024 -smp 4 -nographic -kernel ./nuttx
+
+    NuttShell (NSH) NuttX-10.2.0
+    nsh> uname -a
+    NuttX 10.2.0 31283faf71 Mar  1 2022 19:52:48 arm sabre-6quad
+    nsh> ps
+    PID GROUP PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK   STACK   USED  FILLED COMMAND
+      0     0   0 FIFO     Kthread N-- Ready              00000000 002024 000984  48.6%  Idle Task
+      1     1 100 RR       Task    --- Running            00000000 002016 001232  61.1%  /system/bin/init
+    nsh> free
+                       total       used       free    largest  nused  nfree
+            Umem:    1048224       3728    1044496    1038304      6      2
+            Kmem: 1065201424      10720 1065190704 1065190704     30      1
+            Page:  134217728    1122304  133095424  133095424
+    nsh> /system/bin/hello
+    Hello, World!!
+    nsh> /system/bin/getprime
+    Set thread priority to 10
+    Set thread policy to SCHED_RR
+    Start thread #0
+    thread #0 started, looking for primes < 10000, doing 10 run(s)
+    thread #0 finished, found 1230 primes, last one was 9973
+    Done
+    /system/bin/getprime took 1850 msec

boards/arm/imx6/sabre-6quad/configs/knsh/defconfig

@@ -0,0 +1,83 @@
+#
+# This file is autogenerated: PLEASE DO NOT EDIT IT.
+#
+# You can use "make menuconfig" to make any modifications to the installed .config file.
+# You can then do "make savedefconfig" to generate a new defconfig file that includes your
+# modifications.
+#
+# CONFIG_NSH_CMDOPT_HEXDUMP is not set
+CONFIG_ARCH="arm"
+CONFIG_ARCH_ADDRENV=y
+CONFIG_ARCH_BOARD="sabre-6quad"
+CONFIG_ARCH_BOARD_SABRE_6QUAD=y
+CONFIG_ARCH_BUTTONS=y
+CONFIG_ARCH_CHIP="imx6"
+CONFIG_ARCH_CHIP_IMX6=y
+CONFIG_ARCH_CHIP_IMX6_6QUAD=y
+CONFIG_ARCH_DATA_NPAGES=256
+CONFIG_ARCH_DATA_VBASE=0x80100000
+CONFIG_ARCH_HEAP_NPAGES=256
+CONFIG_ARCH_HEAP_VBASE=0x80200000
+CONFIG_ARCH_INTERRUPTSTACK=2048
+CONFIG_ARCH_IRQBUTTONS=y
+CONFIG_ARCH_KERNEL_STACKSIZE=3072
+CONFIG_ARCH_LOWVECTORS=y
+CONFIG_ARCH_PGPOOL_MAPPING=y
+CONFIG_ARCH_PGPOOL_PBASE=0x18000000
+CONFIG_ARCH_PGPOOL_SIZE=134217728
+CONFIG_ARCH_PGPOOL_VBASE=0x18000000
+CONFIG_ARCH_STACKDUMP=y
+CONFIG_ARCH_TEXT_NPAGES=256
+CONFIG_ARCH_TEXT_VBASE=0x80000000
+CONFIG_ARM_SEMIHOSTING_HOSTFS=y
+CONFIG_BOARD_LOOPSPERMSEC=99369
+CONFIG_BOOT_RUNFROMSDRAM=y
+CONFIG_BUILD_KERNEL=y
+CONFIG_DEBUG_ASSERTIONS=y
+CONFIG_DEBUG_FEATURES=y
+CONFIG_DEBUG_SYMBOLS=y
+CONFIG_DEV_ZERO=y
+CONFIG_DISABLE_ENVIRON=y
+CONFIG_ELF=y
+CONFIG_EXAMPLES_HELLO=m
+CONFIG_FS_HOSTFS=y
+CONFIG_FS_PROCFS=y
+CONFIG_HAVE_CXX=y
+CONFIG_HAVE_CXXINITIALIZE=y
+CONFIG_IDLETHREAD_STACKSIZE=2048
+CONFIG_IMX6_DDRCS_PGHEAP_OFFSET=0x08000000
+CONFIG_IMX6_DDRCS_PGHEAP_SIZE=134217728
+CONFIG_IMX6_UART1=y
+CONFIG_IMX_DDR_SIZE=1073741824
+CONFIG_INIT_FILEPATH="/system/bin/init"
+CONFIG_INIT_MOUNT=y
+CONFIG_INIT_MOUNT_DATA="fs=../apps"
+CONFIG_INIT_MOUNT_FLAGS=0x1
+CONFIG_INIT_MOUNT_FSTYPE="hostfs"
+CONFIG_INIT_MOUNT_SOURCE=""
+CONFIG_INIT_MOUNT_TARGET="/system"
+CONFIG_INIT_STACKSIZE=3072
+CONFIG_INTELHEX_BINARY=y
+CONFIG_LIBC_EXECFUNCS=y
+CONFIG_MM_PGALLOC=y
+CONFIG_NSH_ARCHINIT=y
+CONFIG_NSH_FILEIOSIZE=512
+CONFIG_NSH_FILE_APPS=y
+CONFIG_NSH_READLINE=y
+CONFIG_PREALLOC_TIMERS=4
+CONFIG_RAM_SIZE=1073741824
+CONFIG_RAM_START=0x10000000
+CONFIG_RAM_VSTART=0x10000000
+CONFIG_RAW_BINARY=y
+CONFIG_READLINE_CMD_HISTORY=y
+CONFIG_RR_INTERVAL=200
+CONFIG_SCHED_WAITPID=y
+CONFIG_STACK_COLORATION=y
+CONFIG_START_MONTH=3
+CONFIG_START_YEAR=2022
+CONFIG_SYMTAB_ORDEREDBYNAME=y
+CONFIG_SYSLOG_TIMESTAMP=y
+CONFIG_SYSTEM_NSH=y
+CONFIG_SYSTEM_NSH_PROGNAME="init"
+CONFIG_TESTING_GETPRIME=y
+CONFIG_UART1_SERIAL_CONSOLE=y