redhat/Makefile: fix default values for dist-brew's DISTRO and DIST

In dist-brew target, target-specific variables to change the default value of DISTRO and DIST to 'rhel' and '.el9', but they're not useful because the rest of variables that depends on them, such us BUILD_TARGET, has been calculated yet.

The most visible error is that make dist-brew fails with 'brew: error: No such build target: rawhide'. Rawhide is the default BUILD_TARGET for Fedora, not for RHEL.

Fix it by recursively calling make so the sub-make has the correct DISTRO and DIST values since the beginning, thus calculating the rest of values correctly. Variables BUILD_TARGET and BUILD_PROFILE need to be cleared so they're recalculated in the 2nd level sub-make.

However, variables listed in Makefile.variables file must be allowed to be customizable by the user, so we can't override the ones defined by the user. Save them in variables *_USERDEF so we know if they were defined by user or by the Makefile's logic.

Also, fix a missing closing quote in ifeq ("$(DIST)", ".eln).

Signed-off-by: Íñigo Huguet ihuguet@redhat.com