[gentoo-user] Upgrading kernel with gcc6... use mrproper and not clean

[Daniel Frey <djqfrey@gmail.com>]

I just thought I'd send this out in case others aren't aware...

I've updated some seven machines now to the new profile and gcc6. Out of 
the seven, four of them had problems rebuilding the kernel which led to 
an outright hang during booting (black screen, no kernel panic, just hang.)

I found out `make clean` is not enough, I had to backup my .config, use 
`make mrproper`, restore the .config and run `make silentoldconfig` 
before compiling the kernel and modules.

I am thinking `make clean` didn't remove everything made during the 
compile process and caused some strange race condition somewhere while 
booting (completely guessing here.)

I went back to the three that seemed to be working fine and did the 
mrproper process on them, because if something happens down the line I 
probably won't remember what caused the boot issue.

Dan