Include strace utility by default

Reviewed By: dustin

The strace(1) utility is very handy when developing and debugging
programs.

As a byproduct of compiling the toolchain using crosstool-ng we have a
cross compiled version of strace available.  This patch adds the
strace utility to the final sysroot image by default.

Individual machines can disable this behaviour by putting the
following into their machine.make file:

  STRACE_ENABLE = no

Testing Done:

Compiled ONIE images for powerpc and x86 machines and exercised the
strace utility.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>