Allow this file to be included

Use simple "foo" rather than "${.CURDIR}/foo" to include Makefile.fat
since the former works when including this Makefile from else
where. Also, use full path from ${BOOTSRC} to the FAT templates for
similar reasons. It doesn't change anything in base FreeBSD, but
allows us to have a custom boot1.efi more easily (though that will be
short-lived for us, it may also be helpful for others).

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