mkarchiso: increase the additional free space added to efiboot.img

Less than 1 MiB (but rounded up to one) is needed to account for file and directory entries, especially when using FAT32. Otherwise the ISO build could stop with:

Disk full

The rest of the increase is to allow more space for adding custom files after the fact, i.e. when repacking the ISO. E.g. for the purposes of adding a signed boot loader for Secure Boot or similar. See https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot#ISO_repacking