partition table: Fix regression with make 3.81

"undefine" was added in make 3.8.2

Namespace the global partition table option variables to minimise scope leakage.

components/partition_table/Makefile.projbuild

@@ -8,20 +8,17 @@
 #
 .PHONY: partition_table partition_table-flash partition_table-clean
 
-MD5_OPT :=
+PARTITION_MD5_OPT :=
 ifneq ("$(CONFIG_PARTITION_TABLE_MD5)", "y")
-MD5_OPT := "--disable-md5sum"
+PARTITION_MD5_OPT := "--disable-md5sum"
 endif
 
-FLASHSIZE_OPT :=
+PARTITION_FLASHSIZE_OPT :=
 ifneq ("$(CONFIG_ESPTOOLPY_FLASHSIZE)", "")
-FLASHSIZE_OPT := --flash-size $(CONFIG_ESPTOOLPY_FLASHSIZE)
+PARTITION_FLASHSIZE_OPT := --flash-size $(CONFIG_ESPTOOLPY_FLASHSIZE)
 endif
 
-GEN_ESP32PART := $(PYTHON) $(COMPONENT_PATH)/gen_esp32part.py -q $(MD5_OPT) $(FLASHSIZE_OPT)
-
-undefine FLASHSIZE_OPT  # we don't need these any more, so take them out of global scope
-undefine MD5_OPT
+GEN_ESP32PART := $(PYTHON) $(COMPONENT_PATH)/gen_esp32part.py -q $(PARTITION_MD5_OPT) $(PARTITION_FLASHSIZE_OPT)
 
 # Has a matching value in bootloader_support esp_flash_partitions.h
 PARTITION_TABLE_OFFSET := 0x8000