Add TARGET_FLOSS define

OS_GENERIC is used for host tests and Linux builds but it unfortunately
puts everything at the current working directory. Add a TARGET_FLOSS
define so that we can put them at absolute locations (i.e.
/etc/bluetooth).

Bug: 186442862
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I1c9aaacd1b36b80523437b050d2232b7e5bba47c