Add optimized Makefile support for SoCFPGA handoff

Add optimized Makefile support for SoCFPGA handoff

- Introduce socfpga_g5_handoff_prepare target in U-Boot
  arch/arm/mach-socfpga/config.mk
- Users can convert the handoff via make prepare.
- Detects Altera/Intel SoCFPGA boards from .config
- Combines vendor/board extraction into a single shell call
- Checks for hps_isw_handoff folder and .hiof files
- Uses ls -d instead of find for faster folder detection
- Runs BSP generator script only if files exist
- Non-blocking: continues if handoff folder or files are missing
- HANDOFF_PATH user define allows overriding auto-detected folder
- Minimizes subshells and other slow constructs for faster CI

Signed-off-by: Brian Sune <briansune@gmail.com>