uboot: fix ccache environment and add extension hook

U-Boot build uses `env -i` which clears all environment variables.
CCACHE_DIR and CCACHE_TEMPDIR were not explicitly passed to make,
unlike kernel build (kernel-make.sh). This caused ccache to use
default directory instead of configured Armbian one, breaking
cache statistics and shared cache functionality.

Changes:
- Add CCACHE_DIR and CCACHE_TEMPDIR to uboot_make_envs
- Add uboot_make_config hook for extensions (similar to kernel_make_config),
  allowing modification of environment variables before compilation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: iav

lib/functions/compilation/uboot.sh

@@ -240,10 +240,25 @@ function compile_uboot_target() {
 		"PYTHONPATH=\"${PYTHON3_INFO[MODULES_PATH]}:${PYTHONPATH}\"" # Insert the pip modules downloaded by Armbian into PYTHONPATH (needed e.g. for pyelftools)
 	)
 
+	# Pass the ccache directories explicitly, since we'll run under "env -i"
+	if [[ -n "${CCACHE_DIR}" ]]; then
+		uboot_make_envs+=("CCACHE_DIR='${CCACHE_DIR}'")
+	fi
+	if [[ -n "${CCACHE_TEMPDIR}" ]]; then
+		uboot_make_envs+=("CCACHE_TEMPDIR='${CCACHE_TEMPDIR}'")
+	fi
+
 	# workaround when two compilers are needed
 	cross_compile="CROSS_COMPILE=\"$CCACHE $UBOOT_COMPILER\""
 	[[ -n $UBOOT_TOOLCHAIN2 ]] && cross_compile="ARMBIAN=foe" # empty parameter is not allowed
 
+	call_extension_method "uboot_make_config" <<- 'UBOOT_MAKE_CONFIG'
+		*Hook to customize u-boot make environment*
+		Called right before invoking make for u-boot compilation.
+		Available array to modify:
+		  - uboot_make_envs[@]: environment variables passed via "env -i" (e.g., CCACHE_REMOTE_STORAGE)
+	UBOOT_MAKE_CONFIG
+
 	display_alert "${uboot_prefix}Compiling u-boot" "${version} ${target_make} with gcc '${gcc_version_main}'" "info"
 	declare -g if_error_detail_message="${uboot_prefix}Failed to build u-boot ${version} ${target_make}"
 	do_with_ccache_statistics run_host_command_logged_long_running \