tools: kwbimage: Do not hide usage of secure header under CONFIG_ARMADA_38X

tools: kwbimage: Do not hide usage of secure header under CONFIG_ARMADA_38X

The mkimage host tool can be used to generate kwbimage v1 image with
secure header on host system for A38x plaform also when U-Boot is being
compiled for different platform. So there is no reason to not allow
compiling of mkimage/kwbimage with secure header support for e.g. x86-64
host.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Chris Packham <judge.packham@gmail.com>