[U-Boot] [ANN] U-Boot v2018.03-rc2 released

Hey all,

It's release day and v2018.03-rc2 is out. The big thing is that there's some env related changes that need to be first sorted out and tested, then pulled in.

Other than the env fixes, I hope to pull in some more Kconfig migrations. These I hope will be size neutral, but I know that a few things aren't due to fixing underlying omissions.

Things look on track f26th, -rc4 on March 5th and then release on March 12th. Thanks all!