py, mk: Revert change where build variables set with ?=.

?= operator does not do delayed expansion (unlike =).

py/mkenv.mk

@@ -42,14 +42,14 @@ ECHO = @echo
 CP = cp
 MKDIR = mkdir
 SED = sed
-PYTHON ?= python
-
-AS ?= $(CROSS_COMPILE)as
-CC ?= $(CROSS_COMPILE)gcc
-LD ?= $(CROSS_COMPILE)ld
-OBJCOPY ?= $(CROSS_COMPILE)objcopy
-SIZE ?= $(CROSS_COMPILE)size
-STRIP ?= $(CROSS_COMPILE)strip
+PYTHON = python
+
+AS = $(CROSS_COMPILE)as
+CC = $(CROSS_COMPILE)gcc
+LD = $(CROSS_COMPILE)ld
+OBJCOPY = $(CROSS_COMPILE)objcopy
+SIZE = $(CROSS_COMPILE)size
+STRIP = $(CROSS_COMPILE)strip
 
 all:
 .PHONY: all