Support environmental and command-line `*FLAGS` variable in Makefiles

The Makefiles don't use `CPPFLAGS` or `LDFLAGS`, and set `CFLAGS`
unconditionally.  Rename `CFLAGS`, and add `CPPFLAGS` and `LDFLAGS` to the
compilation and linking recipes in order to support the common patterns of
providing these flags via the command-line and the environment.

Signed-off-by: Jeremy Sowden <azazel@debian.org>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@kernel.org>