Fix parallel make srpm

When building srpm using make -j, each of the recursive makes invoked to
build srpm-{dkms,kmod,utils} will build the dist target. This is both
unnecessary, and also has a very good chance of breaking when they race
trying to build gitrev.

Fix this by make dist a prerequisite of srpm-{dkms,kmod,utils} instead
of srpm-common, so that it will be done once before invoking the
recursive makes.

Also, gitrev is not really required for make dist, so instead of adding
it to BUILT_SOURCES, just add it as a prerequisite of the all target.

Mark the individual package targets as PHONY.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10564

config/deb.am

@@ -1,3 +1,5 @@
+PHONY += deb-kmod deb-dkms deb-utils deb deb-local
+
 deb-local:
 	@(if test "${HAVE_DPKGBUILD}" = "no"; then \
 		echo -e "\n" \