power: Create a standalone ebuild for system_api

system_api is required by a lot of platform2 packages. We need it to be in a
standalone ebuild in order for other packages to depend on it.

This package installs the common headers and compiles the protobufs into static
libraries.

BUG=chromium:386223
TEST=FEATURES=test emerge-amd64-generic platform2.
TEST=trybot run on daisy, link, duck, x86-mario.
TEST=trybot run on lumpy-incremental-release.

CQ-DEPEND=CL:204594

Change-Id: Iabbac86042b30954eac5fc0936804c5c568872bb
Reviewed-on: https://chromium-review.googlesource.com/204583
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/power_manager.gyp b/power_manager.gyp
index fb31b86..502e5e8 100644
--- a/power_manager.gyp
+++ b/power_manager.gyp
@@ -4,13 +4,13 @@
       'deps': [
         'libchrome-<(libbase_ver)',
         'libudev',
+        # system_api depends on protobuf (or protobuf-lite). It must appear
+        # before protobuf here or the linker flags won't be in the right
+        # order.
+        'system_api',
         'protobuf-lite',
       ],
     },
-    'dependencies': [
-      '../system_api/system_api.gyp:system_api-headers',
-      '../system_api/system_api.gyp:system_api-power_manager-protos',
-    ],
     'link_settings': {
       'libraries': [
         '-lgflags',