fix(coreos-postinst): Fix mounting ESP partition
fix(coreos-postinst): Fix mounting ESP partition

Again, goofed up and pipes cannot be used with mapfile (or read) so use
bash process substation instead. Add some safety checks to error if
something goes wrong and there isn't an ESP mount point.

mjg59/update_engine,mjg59/update_engine,endocode/update_engine,mjg59/update_engine,endocode/update_engine,endocode/update_engine