Trying to inherit the STM32F412xE target makes the linker fail, since
__CRASH_DATA_RAM_START__ is not present. Comparing LD scripts with the
STM32F412xG (which has active targets) it seems that the xE variant has
missed some updates somewhere. Since the LD scripts are otherwise
identical, copying the (working) ones from STM32F412xG seems to do the
trick.

Also added flash_data.h which was missing and needed here and there
(copied from xG and updated to fit the xE flash layout).