Don't inherit CFLAGS and LDFLAGS from the environment

kvmtool doesn't build with arbitrary flags, so let's clear CFLAGS and
LDFLAGS by default at the top of the Makefile, allowing people to add
additional options there if they really want to.

Reported by Dave Jones, who ended up passing -std=gnu99 by mistake.

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>