loader: Make EFI entropy size configurable

Add a new loader variable entropy_efi_seed_size which defaults to 2048;
if not defined (e.g. if the /boot/lua/ is updated but /boot/defaults/
isn't) the same 2048 default will be used.

Reviewed by: Val Packett
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46632

(cherry picked from commit f4a69a933cd645e384b337db5ef2ccf41a1ddd5b)