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/20211021093304.25399-1-pali@kernel.org/
Link: https://lore.kernel.org/u-boot/20220111153120.1276641-1-marex@denx.de/
Link: https://lore.kernel.org/u-boot/20230121154743.667253-2-paulerwan.rio@gmail.com/


Cc: Marek Vasut <marex@denx.de>
Cc: Paul-Erwan Rio <paulerwan.rio@gmail.com>
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Simon Glass <sjg@chromium.org>