menuconfig: Add Save/Load buttons

Hi all.

If menuconfig have Save/Load button like alternative .config editors, xconfig, nconfig, etc.We will have a obvious benefit when use menuconfig just like when we use others, we can Save/Load our .config quickly and conveniently.

This patch series add the Save/Load buttons for menuconfig.
It include two patches:
1/2: add Save/Load buttons
2/2: get rid of the top-level entries for "Load an Alternate/Save an Alternate"

Thanks