share/mk/: Unify variable names for regex files

share/mk/: Unify variable names for regex files

Within recipes, variables have delayed expansion, which means that we
can't use variables that we've undefined.  That's why we need to use
a filter to get the regex file name from the rule prerequisites.  This
trick was suggested by Philip.

Cc: Philip Guenther <guenther@gmail.com>
Cc: Paul Smith <psmith@gnu.org>
Cc: Martin Dorey <Martin.Dorey@hitachivantara.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>