make_chroot: clobber existing /etc/mtab
make_chroot: clobber existing /etc/mtab

If our sdk has an /etc/mtab file already, then clobber it.  This fixes
build problems where chromeos-base now installs /etc/mtab for us, but
the sdk build isn't expecting it leading to the error:

INFO    cros_sdk:make_chroot: Running init_setup()...
ln: creating symbolic link `/b/cbuild/new-sdk-chroot/etc/mtab': File exists
Running ['/b/cbuild/src/scripts/sdk_lib/make_chroot.sh', '--stage3_path',
	'/b/cbuild/built-sdk.tar.xz', '--chroot', '/b/cbuild/new-sdk-chroot',
	'--cache_dir', '/b/cbuild/.cache', '--nousepkg'] failed!

BUG=None
TEST=`cros_sdk --chroot foo` still works

Change-Id: I539cf329e93e28534e6ff00577ce415d76918b85
Reviewed-on: https://gerrit.chromium.org/gerrit/43641
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

mjg59/scripts,gtank/scripts,BugRoger/scripts,zhang0137/scripts,vmware/coreos-scripts,marineam/coreos-scripts,gtank/scripts,smilart/scripts,mjg59/scripts,zhang0137/scripts,trnubo/scripts,fivethreeo/scripts,crawford/scripts,BugRoger/scripts,mischief/scripts,nicescale/scripts,coreos/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,endocode/scripts,smilart/scripts,fivethreeo/scripts,vmware/coreos-scripts,fivethreeo/scripts,mjg59/scripts,nicescale/scripts,gtank/scripts,sigma/coreos-scripts,trnubo/scripts,marineam/coreos-scripts,sigma/coreos-scripts,endocode/scripts,endocode/scripts,bcwaldon/coreos-scripts,coreos/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,marineam/coreos-scripts,glevand/coreos--scripts,zhang0137/scripts,crawford/scripts,crawford/scripts,BugRoger/scripts,sigma/coreos-scripts,smilart/scripts,trnubo/scripts,bcwaldon/coreos-scripts,mischief/scripts,mischief/scripts,glevand/coreos--scripts,nicescale/scripts