Save where we're booted from

Record the file path for boot1.efi as the UEFI environemnt variable
FreeBSDBootVarGUID:Boot1Path. Record the device this came from as
FreeBSDBootVarGUID:Boot1Dev. While later stages of the boot may be
able to guess these values by retrieving UEFIGlobal:BootCurrent and
groveling through the correct UEFIGlobal:BootXXXX, this provides
certanty in the face of behavior from any part of the boot loader
chain that might "guess" what to do next. These env variables are
volatile and will disappear on reboot.

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