scripts: add kernel build script using tuxmake for local Linux repo

This script provides an example of how to build a Linux kernel using
tuxmake, targeting both CI and local developer workflows. It assumes
a locally available Linux source tree, the location of which can be
configured using the LINUX_REPO_PATH environment variable.

It sets default values for architecture, toolchain, config, and output
paths, while allowing overrides for custom configurations. It selects
the tuxmake runtime based on context: 'null' for CI systems, or
'docker' if available locally.

The script ensures a clean build environment by removing untracked
files, then invokes tuxmake to build the kernel, modules, and config.
If an Image.gz is generated, it is uncompressed to produce an Image
file suitable for booting.

Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>