manibuilder: Adapt TEST_REMOTE and TEST_REVISION defaults

If ${TEST_LOCAL} is set, adapt the default remote and revision
accordingly. Now this variable is all that is needed to test
a local flashrom commit, e.g.

  $ make alpine:amd64-v3.7 TEST_LOCAL=~/flashrom

Change-Id: I435beb81fd05db25fa6b0a7b517d4c8b9b57ceea
Signed-off-by: Nico Huber <nico.h@gmx.de>
1 file changed