Traditionally, we've used /boot/msdos for the MBR mount point for the SD
images that we produce. For GPT and bsdinstall, we use
/boot/efi. Migrate to using /boot/efi here too, and add a /boot/msdos ->
/boot/efi symlink for compatibility.
We originally used /boot/msdos because originally there was no EFI
support and the FAT partition was needed for uboot.bin, ubldr, and other
SoC specific files. When we added EFI support, we transitioned to just
putting the loader files in the same partition. Later we standardized on
/boot/efi at about the same time we added GPT support to the RE produced
images, but didn't transition the /boot/msdos. Later, we dropped support
of non EFI booting, so the duality of /boot/msdos diminished even
more. Since so little secondary meaning remains, putting it all in
/boot/efi standardizes the location and reflects the RE images
better. We no longer support creating images with ubldr via this
mechanism (though we still support booting that way for legacy users).
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