Migrate build target markers to keyboard.json - Misc (#23609)

keyboards/matrix/m20add/keyboard.json

@@ -34,6 +34,8 @@
     "extrakey": true,
     "rgblight": true
   },
+  "processor": "STM32F411",
+  "bootloader": "custom",
   "layouts": {
     "LAYOUT_tkl_ansi_tsangan": {
       "layout": [

keyboards/matrix/m20add/rules.mk

@@ -1,18 +1,7 @@
-# MCU name
-MCU = STM32F411
-
-# Linker script to use
-# - it should exist either in <chibios>/os/common/ports/ARMCMx/compilers/GCC/ld/
-#   or <this_dir>/ld/
+# custom bootloader
 MCU_LDSCRIPT = m20add_boot
-
-# Board: it should exist either in <chibios>/os/hal/boards/
-#  or <this_dir>/boards
 BOARD = ST_NUCLEO64_F411RE
 
-# Bootloader selection
-BOOTLOADER = custom
-
 CUSTOM_MATRIX = lite
 # project specific files
 SRC += matrix.c tca6424.c rgb_ring.c drivers/led/issi/is31fl3731.c