E.g. specifying 0 (1MB) should reduce 8 GB BAR to 256 MB if 256 MB
BAR is the smallest supported BAR size.

OcBootManagementLib: Set IsExternal from FileSystem->External in custom entries; required for BootEntryProtocol.

OpenLinuxBoot: Allow vmlinuz and initrd paths in *.conf relative to /boot in addition to /, when loader/entries itself is in /boot.
Such paths should be incorrect (and always be relative to /, regardless), but appears to be required to allow booting Linuces which use ostree.

This resolves the general set of compatibility issues with macOS 12
software updater, which requires T2 mac models to have T2 SB models.

This variable is derived from `SecureBootModel` specified in the config
(e.g. j137 becomes J137AP) and is used by macOS 12 during updates
and installations. Setting this variable allows to use macOS 12
with T2 board identifiers, e.g. j137.