Previously, if META values were supplied to the Talos ISO via
environment variable, they will be written down and available after the
install. With this fix, values are also readable and available before
the installation runs (in maintenance mode).
Most of the PR is refactoring `meta.Value(s)` to be a shared library
which is used by the installer/imager and (now) Talos.
Also fixes an issue with not returning properly `NotExist` error when
META is not yet available as a partition on disk.
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
E.g. with the command:
```
make image-metal IMAGER_ARGS="--meta 0xc=abc --meta 0xd=abc"
```
This doesn't support ISO/PXE boot yet, it's going to come into the next
PR.
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
