Increase boot partition size for Arch Linux ARM VM

The 200MB partition is no longer sufficient. When upgrading, I get a:

```
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: '/boot/initramfs-linux-fallback.img'
-> Early uncompressed CPIO image generation successful

gzip: stdout: No space left on device
bsdtar: Write error
bsdtar: Write error
==> ERROR: Initcpio image generation FAILED: 'bsdtar (step 1)' reported an error
error: command failed to execute correctly
```

This would save new users from the need to fiddle with their setup. Make it 500MB like most people would set up for Arch Linux.

Now onto fixing my Arch setup 😄