loader.efi: efipart should be more careful about constructing block device lists

The cd handles should be collected as list of partitions. Some systems also
provide base name for block device (like PciRoot(0x0)/Pci(0x5,0x0)), we need
to be careful about those.

To make out life a bit easier, we prepare not just an array of handles, but we
allocate pdinfo struct for each handle and devicepath, then we can simplify
our work to sort the devices.

Differential Revision: https://reviews.freebsd.org/D21187