The missing "/all" suffix prevents the pc-bios/ parts of the build
from running.

In the meanwhile, -Wall has moved from QEMU_CFLAGS to CFLAGS.  Simplify
everything by not passing down CFLAGS, and add -Wall in the recursive
Makefiles.

Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Fixes: 5e6d1573 ("remove Makefile.target", 2020-08-21)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>