Complete build systems revised

Following #61 (closed), #69 (closed), internal discussions, DI/OT developers meeting #1 and the previous MR !89 (merged), in this MR I present notable improvements to the remaining build infrastructure of the DI/OT System Board boot image repository.

General:

• All build systems are now granular and incremental
• For consistency with !89 (merged), the build systems now contain the cleaning rules: clean, mrproper and distclean.
• Removed reference to absolute paths (variable OUTPUT_DIR) in favor of relative paths PRJ_BUILD_DIR and BUILD_DIR
• All build systems now build to a self-titled subfolder inside the build directory (behavior already present on the gateware subproject)
• All build systems track the output artifacts, opposed to relying on phony targets
• Extensive use of sentinel files to track repository cloning and several stages of project compilation
• Removed BASH_CMD variable
• General cleanup and improvement of the make systems

ATF:

• Use of sentinels to track the cloning of Xillinx ATF repository
• Proper tracking of output artifact

EmbeddedSW:

• Proper tracking of incoming XSA artifact and output FSBL and PMUFW artifacts
• Use of sentinels to track the cloning of Xilinx EmbeddedSW repository
• When XSA is not present, it now triggers gateware generation
• Tracking of FSBL/PMUFW source files for out-of-date compilation detection
• 'Smart' analysis of patch files for both FSBL and PMUFW to resolve files to be patched
• Use of sentinels to track whether sources are in need of patching
• Use of git to reset source files for repatching, if needed

U-boot:

• Proper tracking of source files and DIOT-specific configurations
• Use of sentinels to track the cloning of Xilinx U-boot repository
• 'Smart' analysis of patch files to resolve files to be patched
• Use of sentinels to track whether sources are in need of patching
• Use of git to reset source files for repatching, if needed
• Interface with the internal U-boot build system rules using the top level rules uboot-*
• Slightly modified duplicate copy of the Xillinx device-tree Makefile was converted into a diff patch

Bootbin:

• Proper tracking of incoming artifacts and their dependability on the output artifact
• Capable of calling build system of non-existing incoming artifacts
• Capable of being prompted to call all build system (FORCE flag), even if targets exist (useful for checking if any source of any incoming target was updated)
• All artifacts and targets now reside inside a project build folder

Devicetree:

• Proper tracking of incoming XSA artifact and output device tree artifacts
• When XSA is not present, it now triggers gateware generation
• Use of sentinels on cloning Xilinx device tree repository
• Use of sentinels to track the creation of output device tree artifacts
• All the sources from Xilinx repository and DIOT specifics files are now tracked

CI:

• Bootbin target paths were properly changed to accommodate the change of bootbin target folder