Only fail on known-bad grep configurations.

Rather than exit when any grep configuration is set, only fail on ones we know
to be bad. This lets people use benign configuration options such as
--color=auto and --directories=skip.

Bug: 21260618
Change-Id: I3641c7895df4b43e36a983ea6ad877969cd38bf5

core/config.mk

@@ -35,9 +35,9 @@ backslash := $(patsubst %a,%,$(backslash))
 # only has an effect on python 2.6 and above.
 export PYTHONDONTWRITEBYTECODE := 1
 
-ifneq ($(GREP_OPTIONS),)
+ifneq ($(filter --color=always, $(GREP_OPTIONS)),)
 $(warning The build system needs unmodified output of grep.)
-$(error Please unset your $$GREP_OPTIONS.)
+$(error Please remove --color=always from your  $$GREP_OPTIONS)
 endif
 
 # Standard source directories.