The global doc/linux_sgx.md needs to explicitly describe the methods to
build a debug enclave and hardware running mode. Because using debug key
to signing enclave image rather than production key is still not trivial
in reality.
For the adaption of Inclavare Containers part, add a prolog and give
more details in order to enhance the readability.
Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>
