Makefile: run 'bitbake package-index' after bitbake command building packages

It was recently spotted that during a build of 201705-testing where
sysmocom-openvpn-config was being rebuilt, the Packages(.gz) files
(opkg package indexes) were rebuilt before the new .ipk files for
sysmocom-openvpn-config were written to disk. Also then the Packages
file didn't contain a reference to the new sysmocom-openvpn-config
package version. So, it seems there was a race condition where the
package index was built too early.

Yocto documentation provides some hints:
https://docs.yoctoproject.org/dev-manual/packages.html?highlight=bitbake+package+index#build-considerations
"""
Whenever you perform any sort of build step that can potentially generate
a package or modify existing package, it is always a good idea to
re-generate the package index after the build by using the following
command: bitbake package-index
Do not do "$ bitbake some-package package-index" as BitBake does not schedule
the package index for after the completion of the package you are building.
Consequently, you cannot be sure of the package index including
information for the package you just built. Thus, be sure to run the
package update step separately after building any packages.
"""

So far it seems we were relying on the "create filesystem" task
triggered for image creation which would also trigger the package index
recreation, according to
https://yocto.yoctoproject.narkive.com/rjvLrVpW/question-about-rebuilding-rpm-package-index-for-updated-rpms-when-bitbake-completes
"""
The feed is normally indexed (createrepo) either when you manually run the
package-index operation, or when you construct a filesystem. Until you do that,
the feed directories are transient.
"""

Related: SYS#6023

Makefile

@@ -131,7 +131,7 @@ build.%: | git/poky
 
 # Start a build..
 %-build: | build.$(subst -build,,%) git/poky
-	$(Q)/bin/bash -c "source git/poky/oe-init-build-env build.$(CUR_MACHINE) && bitbake $(BUILD_TARGETS)"
+	$(Q)/bin/bash -c "source git/poky/oe-init-build-env build.$(CUR_MACHINE) && bitbake $(BUILD_TARGETS) && bitbake package-index"
 
 %-upload: | build.$(subst -upload,,%) git/poky
 	$(Q)cd build.$(CUR_MACHINE) && ../scripts/upload-build.sh $(CUR_MACHINE) $(FEED_NAME)