tools: kwbimage: Define structures for extended kwbimage v0 headers

They are used by Marvell Dove 88AP510 BootROM.

After the main header is a list of optional extended headers and after that
is a list of optional binary executable headers. Between each two extended
headers is additional 0x20 byte long padding.

Original Kirkwood SoCs support only one extended header and no binary
executable header.

Extension of struct ext_hdr_v0 is backward compatible with the old
definition. Only reserved[] fields are changed.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>