Read in and parse /efi/freebsd/loader.env from the boot device's
partition as if it were on the command line.

Fetch FreeBSD-LoaderEnv UEFI enviornment variable. If set, read in
loader environment variables from it. Otherwise read in
/efi/freebsd/loader.env. Both are read relative to the device
loader.efi loaded from (they aren't full UEFI device paths)

Next fetch FreeBSD-NextLoaderEnv UEFI environment variable. If
present, read the file it points to in as above and delete the UEFI
environment variable so it only happens once.

This lets one set environment variables in the bootloader.
Unfortunately, we don't have all the mechanisms in place to parse the
file, nor do we have the magic pattern matching in place that
loader.conf has. Variables are of the form foo=bar. No quotes are
supported, so spaces aren't allowed, for example. Also, variables like
foo_load=yes are intercepted when we parse the loader.conf file and
things are done based on that. Since those aren't done here, variables
that cause an action to happen won't work.

Reviewed by: bcran
Differential Revision: https://reviews.freebsd.org/D20016