build: fix ordering of pkgbase targets to get a dtb package reliably

dtb.plist is generated by mtree-to-plist when it processes the kernel
metalog, so we shouldn't try to create dtb packages until that's been
done.  Add another level of indirection to the create-kernel-packages
target to organize all of the kernel flavors under a single target that
is ordered before the create-dtb-packages target.

Note that we may later refine these targets after further pkgbase
review; this is simply a maybe-final stop-gap that's been tested to
fix a known problem.

MFC after:	37 seconds (pkgbase fix for 15.0 builds)
Reviewed and tested by:	cperciva
Differential Revision:	https://reviews.freebsd.org/D53017

Makefile.inc1

@@ -2240,7 +2240,7 @@ _default_flavor=	-default
 _debug=-dbg
 . endif
 
-create-dtb-package:
+create-dtb-package:	.PHONY
 	@if [ -f ${KSTAGEDIR}/${DISTDIR}/dtb.plist ]; then \
 	${SRCDIR}/release/packages/generate-ucl.lua \
 		PKGNAME "dtb" \
@@ -2265,9 +2265,12 @@ create-dtb-package:
 		-o ${REPODIR}/${PKG_ABI}/${PKG_OUTPUT_DIR} ; \
 	fi
 
-create-kernel-packages:	.PHONY
+create-kernel-packages:	.PHONY create-kernel-flavored-packages create-dtb-package
+create-kernel-flavored-packages:	.PHONY
+.ORDER: create-kernel-flavored-packages create-dtb-package
+
 . for flavor in "" ${_debug}
-create-kernel-packages: create-kernel-packages-flavor${flavor:C,^""$,${_default_flavor},} create-dtb-package
+create-kernel-flavored-packages: create-kernel-packages-flavor${flavor:C,^""$,${_default_flavor},}
 create-kernel-packages-flavor${flavor:C,^""$,${_default_flavor},}: _pkgbootstrap .PHONY
 	@cd ${KSTAGEDIR}/${DISTDIR} ; \
 	${METALOG_SORT_CMD} ${KSTAGEDIR}/kernel.meta | \