fix(make.conf): Disable force-mirror

This feature forces emerge to only fetch sources from mirrors, never the
SRC_URI provided in actual ebuilds. Disabling this should fix our issues
with portage tarballs vanishing. :-D
Merge pull request #22 from marineam/mirror

fix(make.conf): Disable force-mirror
Merge pull request #21 from gregkh/kernel_slimdown

fix(cros-kernel/x86_64_defconfig) disable ext3 and ext2