Fix only first BUILD_TARGET element being taken (now correct fix)

Commit d4260e1a4a fixed the issu but only
accidentally. It seems when in a Makefile you do A="xyz abc", the full token
"zyx abc" (quotes included) is subsituted by $(A). As a result, Previous
commit attempting the fix was ending up in ""xyz abc"" being used.

Let's instead remove quotes altogether.

Makefile

@@ -29,14 +29,14 @@ 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-core-image sysmocom-nitb-image sysmocom-core-rauc-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 sysmocom-nitb-image"
-BUILD_TARGET_sysmocom-apu2 = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-image core-image-minimal-initramfs"
-BUILD_TARGET_sysmocom-alix = "core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-image core-image-minimal-initramfs"
+BUILD_TARGET_sysmobts = meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-image sysmocom-core-rauc-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 sysmocom-nitb-image
+BUILD_TARGET_sysmocom-apu2 = core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-image core-image-minimal-initramfs
+BUILD_TARGET_sysmocom-alix = core-image-minimal-initramfs meta-toolchain-osmo task-sysmocom-feed sysmocom-core-image sysmocom-nitb-image core-image-minimal-initramfs
 
 # Pick the one depending on $@. Not sure if .SECONDEXPANSION is more
 # approiate here or not.
-BUILD_TARGETS="$(BUILD_TARGET_$(CUR_MACHINE))"
+BUILD_TARGETS=$(BUILD_TARGET_$(CUR_MACHINE))
 
 GIT_BRANCH_POKY		= "$(POKY_RELEASE)"
 GIT_BRANCH_SYSMOCOM_BSP = "$(SYSMOCOM_RELEASE)"