ebtables: fix passing CFLAGS to configure

EBTABLES_K64U32 is a string passed directly to the configure script,
'+=' operator doesn't have any special meaningi inside it, so
CFLAGS+=-DKERNEL_64_USERSPACE_32 is passed to shell, overwriting previous
CFLAGS value.

Replace CFLAGS+="-DKERNEL_64_USERSPACE_32" with -DKERNEL_64_USERSPACE_32
and pass CFLAGS="$(TARGET_CFLAGS) $(EBTABLES_K64U32)".

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>