For the handful of operations which need to be done with /boot
as the pwd, encapsulate them in subshells to ensure the pwd
doesn't unexpectedly change for other operations, as functions
which need to mount/unmount /boot may fail if the pwd isn't root.

Also, set the pwd to root at the start of detect_boot_device as an
added safety measure.

Test: run oem-factory-reset function, ensure it doesn't fail to
detect boot device due to incorrect working directory.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>