Fix no-dot-config targets for u-boot

All u-boot targets were marked as no-dot-config targets, but that breaks
the build because then the script cannot choose how to set environmental
variables like CROSS_COMPILE. Fix by restricting u-boot no-dot-config
targets to %config, %clean and mrproper
Signed-off-by: Grant Likely <grant.likely@arm.com>

top.mk

@@ -87,7 +87,7 @@ help:
 # Detect when mixed targets is specified, and make a second invocation
 # of make so .config is not included in this case either (for *config).
 
-no-dot-config-targets := clean mrproper distclean u-boot/% tfa/%clean devicetree/clean help
+no-dot-config-targets := clean mrproper distclean u-boot/%clean u-boot/mrproper u-boot/%config tfa/%clean devicetree/clean help
 
 config-targets := 0
 mixed-targets  := 0