porting documentation needs adjustment for new uboot.env setup

[Oliver Grawert]

https://developer.ubuntu.com/en/snappy/guides/porting/ still describes the old style uboot setup, this needs updating with the new methods and should get links to the respective code in snappy-hub branches.

[Michael Vogt (mvo)]

The link is dead, I guess we no longer have a porting guide?

[David Callé (davidc3)]

This doc was no longer relevant and very outdated, marking as invalid.

See https://docs.ubuntu.com/core/en/guides/build-device/board-enablement for the new (still incomplete regarding kernel, but with end-to-end examples)