Add apps/examples/slcd, Remove USB from STM32L-Discovery, LSE support for the STM32 L family, some STM32L-Discovery LCD debug changes

Fix STM32L15X vector definitions; Fix STM32L-Discovery LED controls.  The NSH configuration is now fully functional on the STM32L-Discovery board.

Add audio/ sub-syste, include/nuttx/audio.h interface, and drivers/audio/vs1053.c driver to NuttX.  From Ken Pettit

Add kernel build support and a kernel most OS test example for the Mikroe STM32 M4 board.  From Ken Pettit

apps/examples/helloxx entry point name was mangled when built as an NSH builtin; plus some README updates

configs/hymini-stm32v converted nsh and nsh2 configs to use kconfig-frontends tools; Also converted to use the common SSD1289 driver

Default NxWidgets and NxWM priorities should be 100 not 50 to be consistent with other default priorities

configs/mikroe-stm32f4:  Add new configurations plus support for the MIO283QT2 display from Ken Pettit