Deduplication of flash code for STM32F0 and F1.

Extension of code for STM32F1 to allow for dual bank series XL.
Small changes to documentation for F2, F4 and L1 to add a parameter reference.

Tested with STM32F103RBT6
(note: tests show that the PG bit must be cleared after programming, otherwise
a subsequent erase attempt fails. This has been added to flash_program_half_word
for F0 and F1 only. A fix for the other families is not included in this PR.)