Cleanup compiler warning flags in lib/libefivar/Makefile

There is no need to set -Wno-unused-parameter twice, and instead of
appending to CFLAGS, append to CWARNFLAGS instead. While here, add
-Wno-unused-but-set-variable for the sake of clang 13.0.0.

(cherry picked from commit f643997a1761689ef9108d69b86d42881ea0ab1c)