Improve build system robustness

Build system robustness improved by several methods:
* Mark standard 'build' folder as ignored. This helps
when doing different builds from a same folder
* Combine source files for assembler kernels in same
targets
* Add sorting for new kernel lists
* Relax clean step in CI for faster builds

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>