Enhance copy-armbian-kernel script to support copying U-Boot bootloader
packages in addition to kernel packages. Key changes:
- Add COPY_UBOOT flag (default: true) to enable/disable U-Boot copying
- Add UBOOT_SELECT variable for board/branch filtering (e.g., 'odroidm1:current')
- Split JQ program into reusable components (JQ_EXPAND, JQ_KERNEL_PAIRS, JQ_UBOOT_TRIPLETS)
- Add uboot_selected() function for pattern-based U-Boot selection
- Extract copy operations into copy_flat() and copy_recursive() helpers
- Track selected kernel pairs to filter U-Boot packages by default
