STM32F7 has up to 128KiB of DTCM memory that is currently left unused.

This patch adds DTCM to main heap if CONFIG_STM32F7_DTCMEXCLUDE is not enabled.

Add twr-k64f120m config and fix some ENET related problems

Approved-by: Gregory Nutt

Add driver for the ST L3GD20 3 axis gyro

Approved-by: Gregory Nutt

Allow board to configure HSE clock in bypass-mode. This is needed to enable HSE with Nucleo-F746ZG board.

LM3S Ethernet: Fix interrupt work in the last big commit.

Approved-by: Gregory Nutt

C library:  Add ffs().  Add strings.h.  Move strcasecmp, strncasecmp, bzero, bcmp, and bcopy to where they belong in strings.h.h, not string.h.  bzero, bcmp, and bcopy are legacy functions; the contemporary counterparts should be used instead.

Don't build libc/pthread if pthreasa are disable.  Fix a warning in dirent logic if mountpoints are disabled.