cros_workon_make: stop using common.sh

We want to kill off common.sh entirely.  This uses it for a few things:
* shflags
* GCLIENT_ROOT
* DEFAULT_BOARD
* info/warn/error/die helpers

These are simple enough that we can just inline them in the script
to make it standalone.

BUG=None
TEST=`cros_workon_make --board betty dev-install` builds dev-install
TEST=`cros lint` doesn't point out any missing vars

Change-Id: Icce3c3ea199fa54db40b9872f96ef5251028b3f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1843272
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed