Deprecate the support for x86 sdk

OpenBMC as a project has moved away from using yocto x86 sdks for unit
testing. Instead now the preferred way to build and test code is by two
ways:

1. Leveraging the meson subproject support
2. Using openbmc docker container

Since x86 sdks needed some weird hacks in the code base like figuring
out the linker arguments ourselves & running custom bash commands from
the meson build files. Also with changing yocto code base, its a large
maintainer burden to make sure this support works.

Tested:
Code builds & unit tests are passed with both the approaches mentioned
above.

Change-Id: Iaf55e2c003ffd9ee1a295de5bdfd14d81222e94b
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>