[U-Boot] [ANN] U-Boot v2019.04-rc4 released

Hey all,

So it's release day and I've put up v2019.04-rc4, I've updated git and the tarballs are also up now.

Thanks again to having signed tags, between -rc3 and -rc4 we have a good changelog under 'git log --merges v2019.04-rc3..v2019.04-rc4'

Right now I have outstanding changes in u-boot-socfpga and u-boot-usb to apply. I will, I've just been kicking builds on migrating CONFIG_SPL_TEXT_BASE today, and I don't want to hold everything up on being able to test those PRs.

We're looking at release on April 8th, 2019.

Thanks all!