kconfig: mark config as changed when loading an alternate config

Michal Zachar <mgzachar@mail.t-com.sk> reported that
menuconfig did not save the new config when loading
an alternate config unless he altered it manually.

Mark config as changed upon load of alternate config fixed this.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Roman Zippel <zippel@linux-m68k.org>