[l1-flash/eeprom] Add lock/unlock/eeprom helper routines

Any thoughts on this?

Particularly whether to drop the lock_x, unlock_x, lock_y, unlock_y routines and just have the single flash_lock(bitmask) routine?  That could at one point become a common function, if not with common implementations.

Alternatively, to drop the bitmask/generic style, and just keep explicit multi functions.

Second question: should eeprom_xxx routines be in this file, or in another file altogether?

eeprom_program_words() and it's dependents have been used pretty heavily.