Feature request: allow configuring boot partition size for install to-disk

`bootc install to-disk` currently creates a 512mb boot partition. In the age of ballooning initramfs sizes, this might not be enough to fit the two copies of kernel + initramfs required for updates, particularly when Nvidia drivers are involved. Fedora recently upped their recommended boot partition to 2GB (!) because even their previous 1GB default was deemed insufficient.

Allow specifying `--boot-size` in addition to `--root-size`.