scripts: recover on partially downloaded files

Download to an intermediate temp file, and rename it to its final
name only of download succeeds.

This catches both a failed download, and also the case where the user
interrupts the download. Thus, the a partial download gets discarded,
and we no longer try to extract a partial tarball, which we would
previously have done.

Suggested by Thomas PETAZZONI.

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

 # AVR32 specific configuration file

 config ARCH_avr32

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     select ARCH_SUPPORT_FPU

      help

       The AVR32 architecture, as defined by:

         http://www.atmel.com/products/avr32