Allow building content shell when xwalk directory is absent

Allow building content shell when xwalk directory is absent

Currently, it's not possible to build anything if xwalk directory is not
present. It would be great if we could, because xwalk is on a separate
repository, and might not always be available.

This patch sources gyp xwalk related files only if they are to be used.
Instead of passing xwalk.gyp path to gyp_chromiumefl, a flag --xwalk is
now passed. If and only if set, gyp_chromiumefl will source xwalk.gyp
and other xwalk related gyp files.

Change-Id: I872985cbe25a684cb8e325173e21a1a09cff313b
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>