skeleton: use backtick instead of $(shell ...) make function

Only the instance for setting the root password is changed. The
instance in SYSTEM_GETTY is harmless since it just uses echo and tail.

If SKELETON_TARGET_GENERIC_ROOT_PASSWD is passed as a hashed password
by the user, it contains $, so the '-quotes are needed. If it is not,
we need the backtick to be expanded so "-quotes are needed. Therefore,
the quoting is moved to the definition of SYSTEM_ROOT_PASSWORD.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>