tools: Use a single target-independent config to enable OpenSSL

Host tool features, such as mkimage's ability to sign FIT images were
enabled or disabled based on the target configuration. However, this
misses the point of a target-agnostic host tool.

A target's ability to verify FIT signatures is independent of
mkimage's ability to create those signatures. In fact, u-boot's build
system doesn't sign images. The target code can be successfully built
without relying on any ability to sign such code.

Conversely, mkimage's ability to sign images does not require that
those images will only work on targets which support FIT verification.
Linking mkimage cryptographic features to target support for FIT
verification is misguided.

Without loss of generality, we can say that host features are and
should be independent of target features.

While we prefer that a host tool always supports the same feature set,
we recognize the following
  - some users prefer to build u-boot without a dependency on OpenSSL.
  - some distros prefer to ship mkimage without linking to OpenSSL

To allow these use cases, introduce a host-only Kconfig which is used
to select or deselect libcrypto support. Some mkimage features or some
host tools might not be available, but this shouldn't affect the
u-boot build.

I also considered setting the default of this config based on
FIT_SIGNATURE. While it would preserve the old behaviour it's also
contrary to the goals of this change. I decided to enable it by
default, so that the default build yields the most feature-complete
mkimage.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>