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.)
