drivers/timers/arch_timer.c: implement timer arch API on top of timer driver interface
drivers/timers/arch_alarm.c: Implement alarm arch API on top of oneshot driver interface
drivers/timers/arch_rtc.c: Implement RTC arch API on top of RTC driver interface
include/nuttx/timers/rtc.h: Extend struct rtc_time by adding tm_nsec if RTC supporst hiresolution time.
drivers/serial/Kconfig: It is no longer necessary to restrict Ctrl-C handling to the FLAT build
sched/signal: Add a new configuration option to select signal default actions, separate handling of signal default actions from both task startup logic and from the serial TTY Ctrl-C logic. Add a signal set in the group structure to keep track of what signals have been set to the default action. In dispatching signals in PROTECTED or KERNEL mode, use this signal set to determine if the default signal handler is attached and dispatch the signal in kernel mode for the default actions.
driver/ioexpander: Add gpio_pin_unregister function to GPIO driver
driver/ioexpander: Add pinset struct to GPIO driver for interrupt pins larger than 64
driver/ioexpander: Initialize pintype/inttype when registering ioexpander device
driver/ioexpander: Add SETPINTYPE ioctl command to the GPIO driver
drivers/serial/uart_16550.c: Support 16550 auto hardware flow control
drivers/serial/uart_16550.c: Add configuration option CONFIG_16550_SUPRESS_INITIAL_CONFIG. This is identical to the standard configuration in arch/Kconfig CONFIG_SUPPRESS_UART_CONFIG, but with scope of only the 16550 driver.
drivers/serial/uart_16550.c: Add a configuration, analogous to the STM32 configuration option, to suppress the NuttX standard re-ordering for /dev/ttySN for special case of the 16550 UART.
config/serial: UART 16550: Add CONFIG_SERIAL_UART_ARCH_MMIO option so the a memory mapped device doesn't need to provide uart_getreg() and uart_putreg() implementations.
u16550_txempty() should check UART_LSR_TEMT to avoid some data left in the transmit FIFO
drivers/serial: Add Ctrl-C/Ctrl-X support. This is an initial working implementation that still requires that several details be resolved.
sched/task/task_start.c: Add default signal action to SIGKILL.
sched/wqueue: Modify high priority work queue to support multiple threads.
sched/wqueue and libs/libc/wqueue: workqueues don't need set global data to zero since .bss is cleared automatically. Removing this unnecessary initialization also avoids the loss the work items queued before initialization.
