trusty: Add vendor variant of libtrusty_coverage

We want to add coverage statistics to the trusty-ut-ctrl tool, which is
a vendor binary. Thus we need a vendor variant of libtrusty_coverage.
Merges system libtrusty_test and vendor libtrusty into a single
vendor_available library so that we can add vendor_available to
libtrusty_coverage and make it accessible from vendor tools.

Bug: 175221942
Test: make libtrusty_coverage
Change-Id: I68cc8f1c1580bda8591dbe744e9751474811576d