crossgcc: improve error message on missing tools

People were confused about the 'missing toolchain', so
improve the error message.

Change-Id: Icaee338aeedce2255bcfdafe5407c9df02ad9c4a
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10036
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>

util/crossgcc/buildgcc

@@ -125,7 +125,7 @@ searchtool()
 			fi
 		fi
 	fi
-	printf "${RED}ERROR:${red} Missing toolchain: $1${NC}\n" >&2
+	printf "${RED}ERROR:${red} Missing tool: Please install $1 (eg using your OS packaging system)${NC}\n" >&2
 	[ -z "$3" ] && exit 1
 	false
 }