Fix out of tree builds

[mid-kid]

This allows the build directory to be located outside of the source tree. Arguably, this has no tangible benefit, but it's more "proper" in the meson spirit.

Depends on the following PRs:

• https://github.com/pret/pokeplatinum/pull/155
• https://github.com/Nomura-RH/libvct/pull/1
• https://github.com/Nomura-RH/NitroDWC/pull/2
• https://github.com/Nomura-RH/NitroSDK/pull/1
• https://github.com/Nomura-RH/NitroWiFi/pull/1