We have
o CFLAGS_KERNEL: Additional flags for files compiled into vmlinux
o AFLAGS_KERNEL: Additional flags for files assembled into vmlinux
o MODFLAGS: Additonal flags for files compiled as module

For consistency, rename MODFLAGS to CFLAGS_MODULE, introduce
AFLAGS_MODULE but leave it empty for now.

While we're at, removing "MAKING_MODULES=1", nobody is using it.