tools: kwbimage: Fix invalid UART kwbimage v1 headersz

Armada 385 BootROM ignores low 7 bits of headersz when parsing kwbimage
header of UART type, which effectively means that headersz is rounded down
to multiply of 128 bytes. For all other image types BootROM reads and use
all bits of headersz. Therefore fill into UART type of kwbimage v1 headersz
aligned to 128 bytes.

Fixes: 2b0980c2 ("tools: kwbimage: Fill the real header size into the main header")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>