Cleanup build scripts and add success message

Make the configuration options more readable, factorize some code, and
indicate successful installation of packages.
Use same compilation scheme for Linux and Nouveau

Use the same compilation scheme as other packages for Linux and Nouveau,
namely:

- build into out/build/
- perform installation within build script
- install into $SYSROOT
- use the downloaded cross-compilation toolchain

Result boots and is usable, yay!
linux: preserve L4T kernel and DTBs, generate u-boot script
Only set CROSS_COMPILE environment variable if not present