Add source_root parameter to bluetooth_packetgen

In packetgen, add a source-root parameter that we use to rebase paths
from. This was necessary to get the same directory output structure on
Linux builds with GN.

Bug: 176847256
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I75cdb1693655356ad1470d29a8a45440c775df8f