Export the subproject into config/chroot and config/binary, so that this

information is available to per-project hooks that need to be
subproject-aware (e.g., to skip steps when SUBPROJECT=minimize)

live-build/auto/config

@@ -687,8 +687,10 @@ lb config noauto \
 	"$@"
 
 echo "LB_CHROOT_HOOKS=\"$CHROOT_HOOKS\"" >> config/chroot
+echo "SUBPROJECT=\"$SUBPROJECT\"" >> config/chroot
 echo "LB_BINARY_HOOKS=\"$BINARY_HOOKS\"" >> config/binary
 echo "BUILDSTAMP=\"$NOW\"" >> config/binary
+echo "SUBPROJECT=\"$SUBPROJECT\"" >> config/binary
 
 case $ARCH+$SUBARCH in
 	armhf+raspi2)