platform2: Update to build against libchrome-242728 by default

Make platform2 to build against libchrome-242728 by default and remove
the libbase_ver override in gyp files.

BUG=chromium:331128,chromium:361748
CQ-DEPEND=CL:194475
TEST=Tested the following:
1. `FEATURES=test emerge-$BOARD platform2`
2. `FEATURES=test USE='attestation' emerge-$BOARD platform2`
2. `FEATURES=test USE='buffet' 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/194494
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>

buffet/buffet.gyp

diff --git a/buffet/buffet.gyp b/buffet/buffet.gyp
index 63d8d62..39bdc65 100644
--- a/buffet/buffet.gyp
+++ b/buffet/buffet.gyp
@@ -1,7 +1,4 @@
 {
-  'variables': {
-    'libbase_ver': 242728,
-  },
   'target_defaults': {
     'dependencies': [
       '../../metrics/libmetrics-<(libbase_ver).gyp:libmetrics-<(libbase_ver)',