As commit 3ce1e4c3d3 ("d1: define subtarget specifically") added the
'generic' subtarget, without 'BUILD_SUBTARGET' the correspond U-Boot
package will be no longer selected automatically.
Fixes: 3ce1e4c3d3 ("d1: define subtarget specifically")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>