Build core images instead of nitb images

We don't plan to support NITB or old openbsc repo in 201705 for now.
Build the core images instead of the NITB, which since recently don't
contain osmo-nitb or other binaries from the openbsc repo, only osmo-pcu
and osmo-bts are included.

Makefile

@@ -29,10 +29,10 @@ MACHINES=sysmobts sysmobts2100 sysmocom-apu2 sysmocom-alix
 FEED_NAME=$(SYSMOCOM_RELEASE)-testing
 
 # The default targets to pick depending on machine
-BUILD_TARGET_sysmobts = "meta-toolchain-osmo task-sysmocom-feed sysmocom-nitb-image sysmocom-nitb-rauc-image image-rauc-rescue-initramfs image-rauc-slot-initramfs image-rauc-ubi"
+BUILD_TARGET_sysmobts = "meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-rauc-image image-rauc-rescue-initramfs image-rauc-slot-initramfs image-rauc-ubi"
 BUILD_TARGET_sysmobts2100 = "meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image"
-BUILD_TARGET_sysmocom-apu2 = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-nitb-image core-image-minimal-initramfs"
-BUILD_TARGET_sysmocom-alix = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-nitb-image core-image-minimal-initramfs"
+BUILD_TARGET_sysmocom-apu2 = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image core-image-minimal-initramfs"
+BUILD_TARGET_sysmocom-alix = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image core-image-minimal-initramfs"
 
 # Pick the one depending on $@. Not sure if .SECONDEXPANSION is more
 # approiate here or not.