samd: Make common linker scripts, rename board.mk to mpconfigboard.mk.

The rename matches other ports, e.g. stm32, and gives consistency with
mpconfigboard.h.

ports/samd/Makefile

@@ -10,7 +10,7 @@ $(error Invalid BOARD specified: $(BOARD_DIR))
 endif
 
 include ../../py/mkenv.mk
-include $(BOARD_DIR)/board.mk
+include $(BOARD_DIR)/mpconfigboard.mk
 
 # Qstr definitions (must come before including py.mk)
 QSTR_DEFS = qstrdefsport.h

ports/samd/boards/ADAFRUIT_ITSYBITSY_M4_EXPRESS/board.mk → ports/samd/boards/ADAFRUIT_ITSYBITSY_M4_EXPRESS/mpconfigboard.mk

 MCU_SERIES = SAMD51
 CMSIS_MCU = SAMD51G19A
-LD_FILES = $(BOARD_DIR)/link.ld sections.ld
+LD_FILES = boards/samd51g19a.ld sections.ld
 TEXT0 = 0x4000

ports/samd/boards/ADAFRUIT_TRINKET_M0/board.mk → ports/samd/boards/ADAFRUIT_TRINKET_M0/mpconfigboard.mk

 MCU_SERIES = SAMD21
 CMSIS_MCU = SAMD21E18A
-LD_FILES = $(BOARD_DIR)/link.ld sections.ld
+LD_FILES = boards/samd21x18a.ld sections.ld
 TEXT0 = 0x2000