build: Test against mcu,level,target vars where possible, for clarity

src/Makefile

@@ -18,17 +18,17 @@ OBJS += volume.o
 OBJS-$(debug) += console.o
 OBJS-$(logfile) += logfile.o
 
-ifeq ($(bl_update),y)
+ifeq ($(target),bl_update)
 
 OBJS += bl_update.o
 SUBDIRS += display gotek
 
-else ifeq ($(io_test),y)
+else ifeq ($(target),io_test)
 
 OBJS += io_test.o
 SUBDIRS += display gotek
 
-else ifeq ($(bootloader),y)
+else ifeq ($(target),bootloader)
 
 OBJS += fw_update.o
 SUBDIRS += display fatfs gotek usb

src/display/Makefile

@@ -3,7 +3,7 @@ OBJS += lcd_$(mcu).o
 OBJS += oled_font_6x13.o
 OBJS += led_7seg.o
 
-ifneq ($(bootloader),y)
+ifneq ($(target),bootloader)
 lcd_$(mcu).o: CFLAGS += -Dfont_extra=1
 OBJS += oled_font_8x16.o
 endif

src/image/Makefile

@@ -3,7 +3,7 @@ OBJS += image.o
 
 OBJS-$(quickdisk) += qd.o
 
-ifeq ($(apple2),y)
+ifeq ($(target),apple2)
 shugart := y
 endif