h8300: Don't set CROSS_COMPILE unconditionally

The h8300 architecture Makefile sets the CROSS_COMPILE variable to
"h8300-unknown-linux-" unconditionally but doing that prevents the
variable to be used if a toolchain with a different binary name is
installed in the machine.

So only set CROSS_COMPILE to that default if the variable has not
been set by the user before.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>

arch/h8300/Makefile

@@ -22,7 +22,9 @@ KBUILD_CFLAGS += -DUTS_SYSNAME=\"uClinux\"
 KBUILD_AFLAGS += $(aflags-y)
 LDFLAGS += $(ldflags-y)
 
+ifeq ($(CROSS_COMPILE),)
 CROSS_COMPILE := h8300-unknown-linux-
+endif
 
 core-y	+= arch/$(ARCH)/kernel/ arch/$(ARCH)/mm/
 ifneq '$(CONFIG_H8300_BUILTIN_DTB)' '""'