Detect EFI system partition when upgrading boot loader

The installer tries to detect the boot type of the existing pool for in-place upgrades, however this check is incorrect for the new bsdloader installations. As a result, on a UEFI installation the EFI system partition will be modified into an incorrectly sized freebsd-boot partition and stamped with legacy BIOS boot code. The excessive size of the now freebsd-boot partition causes the PMBR boot code to reject the next stage of the boot loader, displaying the message "Boot loader too large" and halting the boot process.