scripts: bossac: add support for bossac 1.9.1

BOSSA 1.7 has built-in bootloader offset handling while 1.9.1
requires the user to supply the offset.  Add support for both by
sniffing the help output and warn the user if the flags needs adding.

Related to #22062

Signed-off-by: Michael Hope <mlhx@google.com>