devserver.py: Create pkgroot symlink

A previous CL (crrev.com/c/2270639) removed the creation of symlink
static/pkgroot -> /build from the makefile and that cause failures in vm
tests. This CL puts back that symlink dynamically when the devserver
comes up.

BUG=chromium:1101186
TEST=./devinstall_test.py reef ~/trunk/src/build/images/reef/latest/chromiumos_test_image.bin

Change-Id: I68bc0babb668ab0589245f30a298c37e1bb1a9f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2283846
Tested-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Tzung-Bi Shih <tzungbi@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/devserver.py b/devserver.py
index ed02309..8a71e14 100755
--- a/devserver.py
+++ b/devserver.py
@@ -1305,6 +1305,9 @@
   else:
     os.makedirs(cache_dir)
 
+  pkgroot_dir = os.path.join(options.static_dir, 'pkgroot')
+  common_util.SymlinkFile('/build', pkgroot_dir)
+
   _Log('Using cache directory %s' % cache_dir)
   _Log('Serving from %s' % options.static_dir)