Makefile: document savedefconfig in 'make help', like it's done in Linux

	@echo '  oldconfig              - resolve any unresolved symbols in .config'

	@echo '  randconfig             - New config with random answer to all options'

	@echo '  defconfig              - New config with default answer to all options'

	@echo '  savedefconfig          - Save current config as ./defconfig (minimal config)'

	@echo '  allyesconfig           - New config where all options are accepted with yes'

	@echo '  allnoconfig            - New config where all options are answered with no'

	@echo '  randpackageconfig      - New config with random answer to package options'