tools: kwbimage: Allow disabling build on non-mvebu platforms

tools: kwbimage: Allow disabling build on non-mvebu platforms

Some users want to build with CONFIG_TOOLS_LIBCRYPTO disabled, which in
general is possible for at least some boards.  32-bit mvebu however
requires kwbimage for building SPL, and kwbimage has a hard dependency
to host OpenSSL.

The new symbol CONFIG_TOOLS_KWBIMAGE allows disabling kwbimage build on
non-mvebu platforms, and thus building without host libcrypto from
OpenSSL.

Based on previous work and discussions, see links below.

Link: https://lore.kernel.org/u-boot/[email protected]/
Link: https://lore.kernel.org/u-boot/[email protected]/
Link: https://lore.kernel.org/u-boot/[email protected]/
Cc: Marek Vasut <[email protected]>
Cc: Paul-Erwan Rio <[email protected]>
Signed-off-by: Alexander Dahl <[email protected]>
Reviewed-by: Simon Glass <[email protected]>