skeleton-common: rename SKELETON_ROOT_PASSWORD to SKELETON_COMMON_ROOT_PASSWORD

skeleton-common: rename SKELETON_ROOT_PASSWORD to SKELETON_COMMON_ROOT_PASSWORD

Package variables should always be prefixed with the package name. For
SKELETON_COMMON_ROOT_PASSWORD, this means we set the variable a second
time based on the value it previously had. Fortunately, this is fine
for make, even for recursively expanded variables.

This also allows to simplify the condition of the empty password - it's
not needed to set it again to empty if it already was empty.

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