This patch makes the build-system integration of noux packages usable
for non-Noux targets. It moves the GNU build system wrapper to
ports/mk/gnu_build.mk, which is now included by noux.mk. This way,
non-noux applications can use the same build-system wrapper. So the
porting of individual applications becomes easier.
This change removes the 'NOUX_' prefix use the various build variables
used by the noux-pkg's target.mk files to steer the behaviour of the GNU
build system. E.g., NOUX_CONFIGURE_ARGS is now called CONFIGURE_ARGS.
Note that there is a single exception to this pattern: The formerly
named NOUX_LIBS is now called LDLIBS because the plain LIBS variable is
used by the Genode build system.
Fixes#2094
