py: Strip leading dirs from frozen mpy files, so any path can be used.

With this patch one can now do "make FROZEN_MPY_DIR=../../frozen" to
specify a directory containing scripts to be frozen (as well as absolute
paths).

The compiled .mpy files are now stored in $(BUILD)/frozen_mpy/.

py/mkrules.mk

@@ -108,11 +108,11 @@ endif
 
 ifneq ($(FROZEN_MPY_DIR),)
 # make a list of all the .py files that need compiling and freezing
-FROZEN_MPY_PY_FILES := $(shell find -L $(FROZEN_MPY_DIR) -type f -name '*.py')
-FROZEN_MPY_MPY_FILES := $(addprefix $(BUILD)/,$(FROZEN_MPY_PY_FILES:.py=.mpy))
+FROZEN_MPY_PY_FILES := $(shell find -L $(FROZEN_MPY_DIR) -type f -name '*.py' -printf '%P\n')
+FROZEN_MPY_MPY_FILES := $(addprefix $(BUILD)/frozen_mpy/,$(FROZEN_MPY_PY_FILES:.py=.mpy))
 
 # to build .mpy files from .py files
-$(BUILD)/$(FROZEN_MPY_DIR)/%.mpy: $(FROZEN_MPY_DIR)/%.py
+$(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py
 	@$(ECHO) "MPY $<"
 	$(Q)$(MKDIR) -p $(dir $@)
 	$(Q)$(MPY_CROSS) -o $@ -s $(^:$(FROZEN_MPY_DIR)/%=%) $^