Add i486 toolchain support

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3349 42af7a65-404d-4744-a932-0658087f49c3

configs/qemu-i486/README.txt

@@ -8,6 +8,7 @@ Contents
 ^^^^^^^^
 
   * QEMU
+  * Toolchains
   * Configurations
 
 QEMU
@@ -51,6 +52,57 @@ Cygwin build problems:
 	   binaries.  I found 0.14.0 here: http://dietpc.org/windows/qemu/, or
 	2. Try building QEMU with MingGW
 
+Toolchains
+^^^^^^^^^^
+
+  Two target environments are supported: (1) Linux and (2) Cygwin under Windows.
+  Any GCC toolchain that can produce i486 ELF binaries should work.  On Linux,
+  you can probably use the installed system gcc.  But that will not work with
+  Cygwin.  Why?  Because the Cygwin gcc (and probably the MinGW gcc as well) do
+  not produce ELF final binaries but, rather, DOS MZ executables (i.e., .exe
+  files).  Those cannot be used with QEMU.
+
+  The file */setenv.sh should be modified to point to the correct path to the
+  GCC toolchain (if different from the default in your PATH variable).
+
+  Cygwin Buildroot Toolchain
+
+  With Cygwin the solution is to build an i486 cross-development toolchain to
+  generate the i486 ELF files needed by QEMU.  The NuttX buildroot package will
+  create such a toolchain.
+
+  NOTE: As of this writing, none of the released buildroot packages support the
+  i486 build.  This is only available in SVN or in any any 1.10 or later buildroot
+  release.
+
+  Buildroot Instructions
+
+  1. You must have already configured Nuttx in <some-dir>/nuttx.
+
+     cd tools
+     ./configure.sh qemu-i486/<sub-dir>
+
+  2. Download the latest buildroot package into <some-dir>
+
+  3. unpack the buildroot tarball.  The resulting directory may
+     have versioning information on it like buildroot-x.y.z.  If so,
+     rename <some-dir>/buildroot-x.y.z to <some-dir>/buildroot.
+
+  4. cd <some-dir>/buildroot
+
+  5. cp configs/i486-defconfig-4.3.3 .config
+
+  6. make oldconfig
+
+  7. make
+
+  8. Edit setenv.h, if necessary, so that the PATH variable includes
+     the path to the newly built binaries.
+
+  See the file configs/README.txt in the buildroot source tree.  That has more
+  detailed PLUS some special instructions that you will need to follow if you
+  run into problems building the toolchain for Cygwin under Windows.
+
 Configurations
 ^^^^^^^^^^^^^^
 

configs/qemu-i486/ostest/ld.script

@@ -33,6 +33,7 @@
  *
  ****************************************************************************/
 
+OUTPUT_ARCH(elf_i386)
 ENTRY(__start)
 SECTIONS
 {