mk: add sensible default target with defconfig

mk: add sensible default target with defconfig

Users can now use 'make defconfig' to generate a configuration using
the most appropriate defaults for the current machine.

<arch-machine-execenv-toolchain>
  arch taken from uname -m
  machine defaults to native
  execenv is taken from uname, Linux=linuxapp, otherwise bsdapp
  toolchain is taken from $CC -v to see which compiler to use

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>