[BFN] Rename variable in Montara platform debian/rules (#5675)

Platforms .deb-package is not installed into the right directory, because variable PLATFORM was defined previously and variable in the makefile is ignored.
https://www.gnu.org/software/make/manual/html_node/Overriding.html

platform/barefoot/sonic-platform-modules-bfn-montara/debian/rules

@@ -1,6 +1,6 @@
 #!/usr/bin/make -f
 
-PLATFORM := x86_64-accton_wedge100bf_32x-r0
+PLATFORM_NAME := x86_64-accton_wedge100bf_32x-r0
 PACKAGE_NAME := sonic-platform-modules-bfn-montara
 SCRIPT_SRC := $(shell pwd)/scripts
 CONFIGS_SRC := $(shell pwd)/configs
@@ -22,10 +22,10 @@ override_dh_auto_install:
 	cp -r $(SCRIPT_SRC)/* debian/$(PACKAGE_NAME)/usr/local/bin
 	dh_installdirs -p$(PACKAGE_NAME) etc/network/interfaces.d/
 	cp -r $(CONFIGS_SRC)/network/interfaces.d/* debian/$(PACKAGE_NAME)/etc/network/interfaces.d/
-	dh_installdirs -p$(PACKAGE_NAME) usr/share/sonic/device/$(PLATFORM)/
-	cp -r $(WHEEL_BUILD_DIR)/*  debian/$(PACKAGE_NAME)/usr/share/sonic/device/$(PLATFORM)/
-	dh_installdirs -p$(PACKAGE_NAME) usr/share/sonic/device/$(PLATFORM)/plugins
-	cp -r $(PLUGINS_DIR)/*  debian/$(PACKAGE_NAME)/usr/share/sonic/device/$(PLATFORM)/plugins/
+	dh_installdirs -p$(PACKAGE_NAME) usr/share/sonic/device/$(PLATFORM_NAME)/
+	cp -r $(WHEEL_BUILD_DIR)/*  debian/$(PACKAGE_NAME)/usr/share/sonic/device/$(PLATFORM_NAME)/
+	dh_installdirs -p$(PACKAGE_NAME) usr/share/sonic/device/$(PLATFORM_NAME)/plugins
+	cp -r $(PLUGINS_DIR)/*  debian/$(PACKAGE_NAME)/usr/share/sonic/device/$(PLATFORM_NAME)/plugins/
 
 override_dh_usrlocal: