crosutils: fix "umount: /tmp/XXX: not mounted" in build_image

crosutils: fix "umount: /tmp/XXX: not mounted" in build_image

The update_bootloaders needs to unmount file system first for syslinux to
manipulate the raw device file, but the real "umount+losetup -d" in
cleanup stage would cause failure.

This CL remounts partition again so that cleanup can be executed
successfully.

BUG=chromium-os:9278
TEST=Manually executed ./build_image;
 Before fix: seeing "umount: /tmp/esp.2q8fSn: not mounted" in last page.
 After fix: no such error.
 The output image is verified to be bootable successfully.

Change-Id: I19e2e062ad814e7bcd54777b5c3cee31f2b92e81

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=df4fc21

[Will Drewry, 2010-11-18 09:21:08]

LGTM - thanks!