cpu/nrf52: set RIOTBOOT_LEN to 8k

 - nrf52 flash page is 4k, the bootloader needs to be x2 so slots
  start at the beginning of a page.

cpu/nrf52/Makefile.include

@@ -1,6 +1,11 @@
 export CPU_ARCH = cortex-m4f
 export CPU_FAM  = nrf52
 
+# Slot size is determined by "((total_flash_size - RIOTBOOT_LEN) / 2)". 
+# If RIOTBOOT_LEN uses an uneven number of flashpages, the remainder of the 
+# flash cannot be divided by two slots while staying FLASHPAGE_SIZE aligned.
+RIOTBOOT_LEN ?= 0x2000
+
 # Export internal ROM alignment and slot sizes for bootloader support
 export MCUBOOT_IMAGE_ALIGN = 8
 export MCUBOOT_SLOT0_SIZE = 0x8000