stm32cube: add build support for stm32l4xx

The STM32MICRO files for STM32L4xx were dropped in an earlier commit,
but we cannot make any use of them yet because the Kconfig options
and Makefile support is not there yet. This commit adds the ability
for code to use stm32cube from the stm32l4xx code.

Change-Id: I58e85e12e4b4648dc9a203f70703bd54675c3d28
Signed-off-by: Fabien Parent <fparent@baylibre.com>