definitions.mk: Don't fail if xmllint isn't installed

This is a workaround for xmllint currently not being installed on the
build slaves. It should be reverted when this is fixed, as it eliminates
a check for sane code.

Change-Id: I46eeadebd931ee1b6812a9eb933ee4b56ebd16a7
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	core/definitions.mk