Add build guards for libmincrypt target

This introduces a build variable that lets us control whether mincrypt
will build from system/core or from the new external/libmincrypt
repository. Once both locations are prepared, we'll switch the
variable to build mincrypt from the new location and afterwards remove
the conditionals.

BUG: 27287913
Change-Id: I8360b9f296c8404b6352dc447faabaf31d6ffc9a
1 file changed