flash/nor/stm32f1x: allow write fallback for flash options

Mostly refactoring.

Rename original stm32x_write_block() to stm32x_write_block_async()
as it uses target async algo.

Introduce new stm32x_write_block() and move slow, host controlled
fallback flash write there.

The change allows stm32x_write_options() to use slow flash write fallback.

While on it rename variables where halfword count is stored.

Change-Id: I386ae15cf052b1490461ed8f7eea5b4403d466f7
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6706


Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>