Configure trout HALs with ?=

?= allows a downstream device to override the default trout HALs
without needing to modify trout's .mk files

Since trout is meant as both a reference and a baseline, it should
be as easy as possible to derive downstream devices from it

Bug: 149054714
Test: build, boot, lshal still shows the right components running
Change-Id: I114f3e5d4fd16b189506743a02365002f863c98e