Rules: Check for CROSS_COMPILE to be empty

In some case where CROSS is defined to be empty
we define CROSS_COMPILE ?= CROSS so at this point
it will be defined but will be empty so check
for the same

Signed-off-by: Khem Raj <raj.khem@gmail.com>

Rules.mak

@@ -158,7 +158,7 @@ endif
 comma:=,
 space:= #
 
-ifndef CROSS_COMPILE
+ifeq ($(CROSS_COMPILE),)
 CROSS_COMPILE=$(call qstrip,$(CROSS_COMPILER_PREFIX))
 endif