tools: kwboot: Add check that kwbimage contains DDR init code

Some NOR images may be execute-in-place and do not contain DDR init code in
its kwbimage header. Such images cannot be booted over UART as BootROM
loads them to RAM. Add check that kwbimage contains DDR init code in its
header (either as binary code header or as the simple register-value set).

In some cases it is possible to load very small image into L2SRAM and when
DDR init code is not required. So check for L2SRAM load address and skip
DDR init code check in this case.

Signed-off-by: Pali Rohár <pali@kernel.org>