toaster.bbclass: Separate artifact dump from image file dump
SDK artifacts were being dumped alongside the image file dump.
However, the dump was not being triggered correctly, as it
should be attached as a postfunc to do_populate_sdk, rather than
do_rootfs, as do_rootfs doesn't always fire when populating
the SDK directory.
By adding a postfunc to do_populate_sdk, the postfunc is fired
in the correct situations.
Also fixes the directory which is scanned for SDK artifacts, as
this is different from the image file directory.
[YOCTO #7603]
Signed-off-by: Elliot Smith <elliot.smith@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
