docs/manual: cargo packages depend on host-rustc

docs/manual: cargo packages depend on host-rustc

As Cargo is either installed by host-rust or host-rust-bin, the
Cargo-based packages should depend on host-rustc.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
[Arnout: also select HOST_RUSTC in Config.in]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: David Pierret <david.pierret@smile.fr>
Tested-by: David Pierret <david.pierret@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>