Fix rk3399 PKGBUILD

[asppsa]

This PR makes some changes & fixes to the Arch Linux PKGBUILD for the RK3399:

• Fixes an apparent copy-paste mistake in the package() step;
• Sets -DCMAKE_INSTALL_PREFIX=/usr using CMake, so that the step moving the files from /usr/local is no longer necessary. This also fixes the /etc/binfmt.d/box86.conf file, which otherwise still points to /usr/local/bin/box86.
• Runs cmake as part of the build, instead of in the prepare stage, and runs it using the -B ... -S options, as recommended in https://wiki.archlinux.org/title/CMake_package_guidelines#Specifying_directories
• Removes the -j flag from make, as in Arch Linux this is something that is normally set via MAKEFLAGS in makepkg.conf instead.

I think it could be a good idea to repeat these changes for the other pkgbuild files - the -DCMAKE_INSTALL_PREFIX one in particular.

[ptitSeb]

I think it could be a good idea to repeat these changes for the other pkgbuild files - the -DCMAKE_INSTALL_PREFIX one in particular.

Yes, that would probably be a good idea. Can you do it?

[asppsa]

Yes, that would probably be a good idea. Can you do it?

Yep, sure.