documentation: Build example in QS and poky.ent variable added

documentation: Build example in QS and poky.ent variable added

I changed the example that builds an image so that it uses the
default build directory.  It seems like the natural thing to do.

Also added a new poky.ent variable named OE_INIT_FILE.  This
variable is set to the name of the build environment script

(From yocto-docs rev: f0db49e27e89aefb6d43a0b455c6ecc529399c27)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>