board: amlogic: document alternative libretech-cc installation methods

board: amlogic: document alternative libretech-cc installation methods

As already documented in this README, several binaries must be
glued together in order to boot the device.

Extend the documentation to cover the prebuilt binaries
(saving you the hassle of installing ancient cross-compilers),
and also mention the open source replacements for the encryption
tool (which is especially useful if you want to avoid requiring
32-bit x86 binaries in your build system).

Signed-off-by: Daniel Drake <drake@endlessm.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>