doorlock.rst: add BOSSA as a flash tool

Currently, the instructions tell to use west, however there was raised issue that instructions are not clear. For me, flashing with BOSSA worked fine.

Relates to: eclipse/oniro-core/docs#141 (closed)

CC: @fldn

Signed-off-by: Grzegorz Gwozdz grzegorz.gwozdz@huawei.com