cromo: 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: Ia4bce7a7b9876a6a31c37a36bafc49978f6dd9d0
Reviewed-on: https://chromium-review.googlesource.com/204591
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/cromo.gyp b/cromo.gyp
index 83de6de..dd1e2cc 100644
--- a/cromo.gyp
+++ b/cromo.gyp
@@ -62,7 +62,6 @@
       # depending on 'libcromo'.
       'dependencies': [
         '../../platform2/common-mk/external_dependencies.gyp:modemmanager-dbus-adaptors',
-        '<(platform_root)/system_api/system_api.gyp:system_api-headers',
         'cromo-adaptors',
       ],
       'defines': [