mkimage: add atmelimage

The new atmelimage converts a machine code BLOB to bootable ROM image. Atmel
ROM has no sophisticated image format, it only checks the first 7 ARM vectors.
The vectors can contain valid B or LDR opcodes, the 6'th vector contains the
image size to load.

Additionally the PMECC header can be written by the atmelimage target. The
parameters must be given via the -n switch as a coma separated list. For
example:

mkimage -T atmelimage \
 -n usePmecc=1,sectorPerPage=4,sectorSize=512,spareSize=64,eccBits=4,eccOffset=36 \
 -d spl/u-boot-spl.bin boot.bin

A provided image can be checked for correct header setup. It prints out the
PMECC header parameters if it has one and the 6'th interrupt vector content.

---8<---
Image Type:	ATMEL ROM-Boot Image with PMECC Header
		PMECC header
		====================
		eccOffset:        36
		sectorSize:      512
		eccBitReq:         4
		spareSize:        64
		nbSectorPerPage:   4
		usePmecc:          1
		====================
		6'th vector has 17044 set
--->8---

A SPL binary modified with the atmelimage mkimage target was succesfully
booted on a sama5d34ek via MMC and NAND.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Bo Shen <voice.shen@atmel.com>
Cc: Heiko Schocher <hs@denx.de>
Tested-by: Bo Shen <voice.shen@atmel.com>