libc/signal and include/signal.h: Rename sigset() to signal().  sigset() is the System V function; signal() is the obsoleted POSIX name.  These seem to be equivalent.  Neither are relevant in modern POSIX programming but the name signal() probably has better credentials.  sigset() is now relegated to a #define in signal.h that makes it equivalent to signal().

Restore top-level .gitignore; testbuild.sh must distinguish CONFIG_ARM_TOOLCHAIN_GNU from other symbols that begin with the same string

Olimex-STM32_H407: Move board initialization logic from stm32_appinit.c to stm32_bringup.c; Add support for the RTC driver

Olimex-STM32-H407:  Pick up a few changes from PR38.  Most of those changes are, however, not appropriate or not incompliance with NuttX naming or coding style.