kbuild: efi: Avoid rebuilding efi targets

Add a couple of missing targets so that helloworld and other efi targets
are not needlessly rebuilt.

CC: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_loader/Makefile b/lib/efi_loader/Makefile
index eff3c25..84d61df 100644
--- a/lib/efi_loader/Makefile
+++ b/lib/efi_loader/Makefile
@@ -17,6 +17,7 @@
 
 ifneq ($(CONFIG_CMD_BOOTEFI_HELLO_COMPILE),)
 always += helloworld.efi
+targets += helloworld.o
 endif
 
 obj-$(CONFIG_CMD_BOOTEFI_HELLO) += helloworld_efi.o