kconfig: rename the defconfig and olddefconfig targets

What 'defconfig' really does is save the current .config to a defconfig,
so it is better named 'savedefconfig' (as other projects do).

What 'olddefconfig' really does is create a .config from a defconfig,
so rename it to 'defconfig' (as other projects do, too).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }