debugd: 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: Ie65994e51a475ebd92262409e11244c401eec78d
Reviewed-on: https://chromium-review.googlesource.com/204603
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/debugd.gyp b/debugd.gyp
index 60279dd..31cf54b 100644
--- a/debugd.gyp
+++ b/debugd.gyp
@@ -135,7 +135,6 @@
           'target_name': 'modem_status',
           'type': 'executable',
           'dependencies': [
-            '<(platform_root)/system_api/system_api.gyp:system_api-headers',
             'debugd-proxies',
           ],
           'sources': [
@@ -170,7 +169,6 @@
           'target_name': 'wimax_status',
           'type': 'executable',
           'dependencies': [
-            '<(platform_root)/system_api/system_api.gyp:system_api-headers',
             'debugd-proxies',
           ],
           'sources': [