samples: use savedefconfig when saving samples

When saving a sample, use savedefconfig instead of copying
the full .config file.

This reduces the saved .config, and reduces clutter when it
is later upgraded.

Also use defconfig when retrieving a sample.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # Compute AVR32-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     CT_TARGET_SYS=none

 }