Add sysroot.bootloader repo config key

Add sysroot.bootloader repo config key

The sysroot.bootloader key configures the bootloader
that OSTree uses when deploying a sysroot. Having this key
allows specifying behavior not to use the default bootloader
backend code, which is preferable when creating a first
deployment from the sysroot (#1774).

As of now, the key can take the values "auto" or "none". If
the key is not given, the value defaults to "auto".

"auto" causes _ostree_sysroot_query_bootloader() to be used
when writing a new deployment, which is the original behavior
that dynamically detects which bootloader to use.

"none" avoids querying the bootloader dynamically. The BLS
config fragments are still written to
sysroot/boot/loader/entries for use by higher-level software.

More values can be supported in future to specify a single
bootloader, different behavior for the bootloader code, or
a list of bootloaders to try.

Resolves: #1774

Closes: #1814
Approved by: jlebon