powertop: new package

[Thomas:
  - fix commit title
  - powertop wants libintl unconditionally, so make sure
    BR2_PACKAGE_GETTEXT is selected when BR2_NEEDS_GETTEXT is set, and
    add gettext to the dependencies.
  - add missing comment about thread dependency.
  - add missing dependency on host-pkgconf, without which powertop
    cannot find libnl.
  - patch src/Makefile.am to not pass -fstack-protector, which fails
    to build if the toolchain does not have SSP support.
  - rename patch powertop-autotune.patch to confirm to the patch
    naming convention.]

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>