scripts: bossac: Support flashing at an offset.

The Adafruit SAMD51 boards use a combined BOSSA / U2F bootloader that
resides in the first 16 KiB.  Add support for pulling the application
partition offset from the configuration and using that to offset where
the binary gets flashed.

Signed-off-by: Michael Hope <mlhx@google.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>