Revert "Makefile: allow overriding CFLAGS on the command line"

Riku Voipio reports a regression introduced by d0e2772b ("Makefile:
allow overriding CFLAGS on the command line"):

| This breaks builds of debian packages as dpkg-buildpackage sets LDFLAGS
| to something unsuitable for guest init.

Revert the problematic patch for the moment, while we rethink how we'd
like to support user-provided toolchain flags.

This reverts commit d0e2772b.

Conflicts:
	Makefile

Signed-off-by: Will Deacon <will.deacon@arm.com>