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: Ic76a747007ba119f6ada750e99d655282bf4e408
Reviewed-on: https://chromium-review.googlesource.com/204602
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/cros-disks.gyp b/cros-disks.gyp
index d7ec212..9e38b55 100644
--- a/cros-disks.gyp
+++ b/cros-disks.gyp
@@ -41,7 +41,6 @@
       'target_name': 'libdisks',
       'type': 'static_library',
       'dependencies': [
-        '<(platform_root)/system_api/system_api.gyp:system_api-headers',
         'libdisks-adaptors',
       ],
       'sources': [