cpu/nrf52: add alignment and image header sizes for bootlaoder usage

cpu/nrf52/Makefile.include

@@ -1,5 +1,11 @@
 export CPU_ARCH = cortex-m4f
 export CPU_FAM  = nrf52
 
+# Export internal ROM alignment and slot sizes for bootloader support
+export IMAGE_ALIGN = 8
+export SLOT0_SIZE = 0x8000
+export SLOT1_SIZE = 0x3C000
+export SLOT2_SIZE = 0x3C000
+
 include $(RIOTCPU)/nrf5x_common/Makefile.include
 include $(RIOTMAKE)/arch/cortexm.inc.mk