loader.efi: Apply command-line DHCP overrides earlier

[kgalazka]

Summary

Ability to override DHCP options with command-line arguments
was affected by intoduction of initmd support. Initmd discovery
configures the network before loader arguments
were parsed, so a dhcp.root-path override was unavailable
during the first network configuration. Move parsing
arguments earlier and apply dhcp.root-path even if DHCP response
does not contain option 17. This allows providing a dynamic NFS
root e.g. by chain loading loader.efi from iPXE.

Signed-off-by: Krzysztof Galazka <[e-mail]>

Sponsored by: Intel Corporation
Assisted by: Github Copilot (GPT-5.6 Sol)

[imp]

The only thing that I worry about in moving this parsing earlier is side effects from env variables taking effect before we have the efi partition list, so things like currdev=disk3p4: stop working. Is that the case?