platform2: Update to build against libchrome-242728 by default

All targets specified in platform2 can now built against
libchrome-242728. This CL flips the default value of BASE_VER from
180609 to 242728.

BUG=chromium:331128,chromium:361748
CQ-DEPEND=CL:194475
TEST=Tested the following:
1. `FEATURES=test emerge-$BOARD platform2`
2. `FEATURES=test USE='wimax gdmwimax' emerge-$BOARD platform2`
3. `FEATURES=test USE='asan clang' emerge-$BOARD platform2`
4. Trybot runs on paladin, release and chromiumos-sdk builders.

Change-Id: I7b49ca84dbddfd5c0e7111aba757f2c005196cdc
Reviewed-on: https://chromium-review.googlesource.com/194476
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
1 file changed