Move the stm32l1 sources, headers and boards into arch/arm/src/stm32l1,
arch/arm/include/stm32l1 and boards/arm/stm32l1, then finalize the split:
source each split family directly in arch/arm/Kconfig and boards/Kconfig and
remove the now-empty combined arch/arm/src/stm32 and boards/arm/stm32 trees.
BREAKING CHANGE: The legacy STM32 architecture and board paths were split into
stm32f1, stm32l1, stm32f2, stm32f3, stm32f4, and stm32g4 directories.
Out-of-tree boards must move from stm32 to the matching split family.
Signed-off-by: raiden00pl <raiden00@railab.me>
