Allow selection of Trusty KeyMint HAL implementation

By default, the existing C++ implementation of KeyMint will continue to
be used.  However, this can be overridden at build time to force use of
the Rust implementation by setting

    export TRUSTY_KEYMINT_IMPL=rust

Note that this requires a concomitant change to the bootloader prebuilts
that include the Trusty prebuilts, to include the corresponding Rust
version of the KeyMint TA.

Bug: 197891150
Bug: 225036046
Test: VtsAidlKeyMintTargetTest
Change-Id: I05b4b7d49cea0ac1c10b3a2e8fa5c49374aa1675