makefile: Use origin check for CC variable

Make defines defaults for commonly used variables, therefore
we need to check first how the variable was defined. If `default`
value was used, override it with our own default.

Makefile

@@ -4,7 +4,9 @@ DEFAULT_CONFIG_LOCATION = 454656
 CONFIG_LOCATION = 458752
 HTML_LOCATION = 262144
 
-CC ?= sdcc
+ifeq ($(origin CC),default)
+CC = sdcc
+endif
 CC_FLAGS = -mmcs51 -I. -Ihttpd -Iuip
 ASM ?= sdas8051
 AFLAGS= -plosgff