Check partition size before flashing boot image in all code paths

While 08d14208 implemented checking if the boot image fits the partition it will be flashed to for the most common flashing code path, there are two other ones that still don't do this check:

1. https://gitlab.postmarketos.org/postmarketos/boot-deploy/-/blob/08d142089900be624755d065a57b6bbbec7e4fa7/boot-deploy-functions.sh#L910-940
2. https://gitlab.postmarketos.org/postmarketos/boot-deploy/-/blob/08d142089900be624755d065a57b6bbbec7e4fa7/boot-deploy-functions.sh#L855-884

We should ensure that all three code paths check this in a consistent way.