Workaround for chromebook 'SeaBIOS' issue.

SeaBIOS of Chromebook C720 reports that there exists a reserved area
around 16MB. The area limits a extended memory area, preventing to
load a kernel image.
This change avoid the problem as DragonFly BSD
(commit 5fc9dc6d407adf96b8a1f7d4b6e7b5f638645a23) does.
http://gitweb.dragonflybsd.org/dragonfly.git/commit/5fc9dc6d407adf96b8a1f7d4b6e7b5f638645a23