initrd: Spawn an emergency shell when something goes wrong

initrd: Spawn an emergency shell when something goes wrong

set -e allows to exit if a command fails. We install a trap and execute
emergency_shell() when either the init script exits or when ctrl-c is
typed (say if we are stuck somewhere and we want to debug it).

(From OE-Core rev: ae5e2bd994e3f60d3803ab56e6ed34d08fbc56f0)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>