redhat/Makefile: fix the detection of "-n" (RHTEST)

Bugzilla: INTERNAL
Upstream Status: RHEL only, https://gitlab.com/cki-project/kernel-ark

Check only the first word of MAKEFLAGS for the "-n" option. Othwerise
the "n" character would be checked against any other parameters passed
on the command line, such as BUILDOPTS and DISTLOCALVERSION.

Reference:
https://www.gnu.org/software/make/manual/make.html#Testing-Flags

NOTE: Without this fix any kernel build command line (make) that contains
the letter "n" in one of its parameters will set RHTEST=1 and skip
the creation of configuration files, which will lead to build
being aborted.

Fixes: 514b856b ("redhat/Makefile: Add RHTEST")
Signed-off-by: Luis Claudio R. Goncalves lgoncalv@redhat.com