OE needs to be able to change the default compiler. If we pass in HOSTCC
through the make command, it overwrites not only this setting but also the 
setting in tools/Makefile wrapped in ifneq ($(CROSS_BUILD_TOOLS),) which 
breaks the build.

We therefore use override to ensure the value of HOSTCC is overwritten when
needed.

RP: Updated the patch to the version being submitted to upstream u-boot

Upstream-Status: Submitted [emailed to Masahiro Yamada for discussion]
RP 2017/3/11

Index: git/tools/Makefile
===================================================================
--- git.orig/tools/Makefile
+++ git/tools/Makefile
@@ -262,7 +262,7 @@ $(LICENSE_H): $(obj)/bin2header $(srctre
 subdir- += env
 
 ifneq ($(CROSS_BUILD_TOOLS),)
-HOSTCC = $(CC)
+override HOSTCC = $(CC)
 
 quiet_cmd_crosstools_strip = STRIP   $^
       cmd_crosstools_strip = $(STRIP) $^; touch $@
Index: git/tools/env/Makefile
===================================================================
--- git.orig/tools/env/Makefile
+++ git/tools/env/Makefile
@@ -8,7 +8,7 @@
 # fw_printenv is supposed to run on the target system, which means it should be
 # built with cross tools. Although it may look weird, we only replace "HOSTCC"
 # with "CC" here for the maximum code reuse of scripts/Makefile.host.
-HOSTCC = $(CC)
+override HOSTCC = $(CC)
 
 # Compile for a hosted environment on the target
 HOST_EXTRACFLAGS  = $(patsubst -I%,-idirafter%, $(filter -I%, $(UBOOTINCLUDE))) \