py/mkenv: Allow providing STRIP as an environment variable.

Allows doing "export STRIP=" in a profile or .envrc file, to
not strip binaries by default.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/mkenv.mk

@@ -52,7 +52,7 @@ GDB = $(CROSS_COMPILE)gdb
 LD = $(CROSS_COMPILE)ld
 OBJCOPY = $(CROSS_COMPILE)objcopy
 SIZE = $(CROSS_COMPILE)size
-STRIP = $(CROSS_COMPILE)strip
+STRIP ?= $(CROSS_COMPILE)strip
 AR = $(CROSS_COMPILE)ar
 WINDRES = $(CROSS_COMPILE)windres