tools: spkgimage: add Renesas SPKG format

Renesas RZ/N1 devices contain BootROM code that loads a custom SPKG
image from QSPI, NAND or USB DFU. Support this format in mkimage tool.

SPKGs can optionally be signed, however creation of signed SPKG is not
currently supported.

Example of how to use it:

tools/mkimage -n board/schneider/rzn1-snarc/spkgimage.cfg \
	-T spkgimage -a 0x20040000 -e 0x20040000 \
	-d u-boot.bin u-boot.bin.spkg

The config file (spkgimage.cfg in this example) contains additional
parameters such as NAND ECC settings.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>