flash/stm32l4x: introduce flash programming without loader

flash/stm32l4x: introduce flash programming without loader

this capability permits to program the flash if we cannot reserve a workarea.

the introduction the command 'stm32l4x flashloader <bank_id> [enable|disable]'
helps to automatically skip using the flashloader if needed.

Change-Id: Id29213c85ee5c7c487cfee21554f5a7ea50db6c9
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6273
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Tested-by: jenkins