pad uefi fat file length

[Freax13]

I got the following error during build:

Error: failed to create UEFI disk image

Caused by:
    No space left on device
thread 'main' panicked at 'build failed', simple_boot/src/main.rs:100:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted (core dumped)

This error occured because the file size of the uefi binary was too close to the created uefi fat file size (in my case the uefi binary was 4184064 bytes and the fat file was 4194304 bytes big). I solved it by adding some padding to make sure that the difference between the binary's size and fat file's size isn't too small. I don't know much about fat so I just guessed the padding. If there's a proper way to calculate the required padding we should probably add that instead.

[phil-opp]

Thank you! I'm not sure if there is a better solution either, so let's just merge this for now.

[phil-opp]

Published together with #182 as v0.10.8