makemake: Switch from os.listdir() to glob.glob() to read SPEC dir

Signed-off-by: Euan Harris <euan.harris@citrix.com>
makemake: Remove some unnecessary duplicate iterations over the specs

Signed-off-by: Euan Harris <euan.harris@citrix.com>
makemake: Remove further unnecessary traversals of the spec list

This change substantially alters the order in which rules appear
in the Makefile.

Signed-off-by: Euan Harris <euan.harris@citrix.com>