netbpfload: enable mainline on API V/35+

netbpfload.mainline.rc is derived from:
  initrc-doc/bpfloader-sdk34-14-U.rc,
except that it is 'override' instead of 'updatable' and uses
the apex netbpfload instead of the platform bpfloader.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1ecf14e3d927ece136a0669a9b209be7112e55b5