Replace bash arithmetic with awk functions

The bash arithmethics in _make_boot_uefi-x64.systemd-boot.esp() introduced rounding issues, that can lead to
insufficient FAT image size for the files.
Conversion functions for awk now replace the bash arithmetics and additionally a ceil() function rounds the calculated
size up to the next full MiB.
Add an info message about the size of the created FAT image.

Fixes #70
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