Bazel support

Information on general Bazel build rules and repository rules.

//asylo/bazel:asylo_deps.bzl. Repository rules for importing dependencies needed for Asylo
//asylo/bazel:dlopen_enclave.bzl. Build rules for the process-separated dlopen enclave backend.
//asylo/bazel:sgx_deps.bzl. Repository rules for importing dependencies needed for the SGX backends
@linux_sgx//:sgx_sdk.bzl. Build rules for defining SGX enclaves.
//asylo/bazel:asylo.bzl. Build rules for defining enclaves and tests.