pkgbase: Create two new rules for creating repo

This adds two new rules named create-packages-kernel-repo and
create-packages-world-repo.
The goal of those rules is to create the {kernel,world} packages and
after that the repository.
It helps a lot for developing with pkgbase by adding the dev machine
repository created by those rules on top of the official pkgbase one.

Differential Revision:	https://reviews.freebsd.org/D43623
Reviewed by:	bapt, emaste
Sponsored by:	Beckhoff Automation GmbH & Co. KG

(cherry picked from commit a5ff900535e24e3a3cd02287d733e1d1e5e9583a)

Makefile

@@ -176,7 +176,9 @@ TGTS=	all all-man buildenv buildenvvars buildetc buildkernel buildworld \
 	builddtb xdev xdev-build xdev-install \
 	xdev-links native-xtools native-xtools-install stageworld stagekernel \
 	stage-packages stage-packages-kernel stage-packages-world stage-packages-source \
-	create-packages-world create-packages-kernel create-packages-source create-packages \
+	create-packages-world create-packages-kernel \
+	create-packages-kernel-repo create-packages-world-repo \
+	create-packages-source create-packages \
 	update-packages packages installconfig real-packages real-update-packages \
 	sign-packages package-pkg print-dir test-system-compiler test-system-linker \
 	test-includes

Makefile.inc1

@@ -2079,6 +2079,10 @@ create-packages-kernel:	_pkgbootstrap _repodir .PHONY
 			SOURCE_DATE_EPOCH=${SOURCE_DATE_EPOCH} \
 			create-kernel-packages
 
+create-packages-kernel-repo: .PHONY create-packages-kernel sign-packages
+
+create-packages-world-repo: .PHONY create-packages-world sign-packages
+
 create-packages-source:	_pkgbootstrap _repodir .PHONY
 	${_+_}@cd ${.CURDIR}; \
 		${MAKE} -f Makefile.inc1 \