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>
diff --git a/platform2.py b/platform2.py
index 3afd8b7..ffc6e2b 100755
--- a/platform2.py
+++ b/platform2.py
@@ -14,7 +14,7 @@
 # Define default the version of libchrome{,os} to build against.
 # Used only if the BASE_VER environment variable is not set.
 # This can also be overridden in a specific target GYP file if required.
-_BASE_VER = '180609'
+_BASE_VER = '242728'
 
 
 class Platform2(object):