Soong: BPF: add defaults support and local include

Add support for defaults clause in the bpf soong module, as well as
local_include_dirs property.

Test: TH
Change-Id: Iaa03e7f1f6dcf0e929d1d5b23bb79ca2a8366bef
Signed-off-by: Neill Kapron <nkapron@google.com>