Get kargs from usr/lib/bootc/kargs.d

[Johan-Liebert1]

Parse toml files in usr/lib/bootc/kargs.d and append them to kernel
cmdline on install and upgrade/switch. Also, copy over current
deployment's cmdline args on upgrade/switch to another deployment

storage: Add boot_dir and esp fields

We have a lot of places where we mount the ESP temporarily and a lot of
switch cases for Grub's vs SystemdBoot's 'boot' directory.

We add a boot_dir field in Storage which points to /sysroot/boot for
systems with Grub as the bootloader and points to the ESP for systems
with SystemdBoot as the bootloader.

Also we mount the ESP temporarily while creating the storage struct,
which cleans up the code quite a bit.