Fix hammerhead backlight (Nexus 5) + remove CONFIG_SERIAL_MSM_CONSOLE

This merge request solves Nexus 5 backlight issue. (see https://gitlab.com/postmarketOS/linux-postmarketos/issues/8#note_105946047 and a picture here https://wiki.postmarketos.org/wiki/File:Nexus-5-backlight-fix.jpg )

It also contains a commit partially reverting commit f2a27fc0 , because it provokes reboot loops when no serial cable is plugged in. The author @drebrez agreed to revert is commit see:
#105 (closed)

I'm doing both in the same merge request for ease (but it contains 2 separated commits), and because it seems useless for me to make a merge request only solving backlight issue, if the kernel is not usable.