Added make vars FLASH_BOOTLOADER_UICR and FLASH_SDK_BOOTLOADER_CONFIG

- You can FLASH_BOOTLOADER_UICR set in your project specific Makefile,
if your project is a bootloader.
  This causes the storage of the bootloader's address to UICR 10001014
so that it is called by the SoftDevice during bootup

- You can FLASH_BOOTLOADER_CONFIG set in your project specific Makefile,
if your project is a normal application and your target system consist
of a bootloader which is based on the Nordic Sdk.
  This causes a write of some configuration data into the last flash
block (typically 16 byte at 3FC00 for a 256k device) that notifies the
bootloader that there is a valid application in the flash.

Currently only implemented for Makefile.posix
Maybe someone like to add windows support.

== !!! There is still one Issue !!! ==
See the FIXME in Makefile.posix.
The bootloader's address is wrote correctly to uicr, but I always get a "Error: Programming failed @ address 0x10001000 (block verification error)"
I don't know how to fix this. Any suggestions?
To not getting always this error message, I'd added a suppression hack for that.