cramfs: add CFLAGS/LDFLAGS and host-zlib dependency

cramfs depends on host-zlib, so it must be built prior to cramfs.

Moreover, we need to pass the HOST_CFLAGS and HOST_LDFLAGS variables,
otherwise it doesn't find zlib in $(HOST_DIR).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>