imx-base.inc: Use specific package feed for MX6QDL and MX6SL

The sharing of packaging feed between MX6QDL and MX6SL is very
fragile. In rare cases, when a package is not generated for the
machine it can fallback to the wrong set of packages.

This has been seen on the Vivante GPU packages as MX6SL does not
include libegles2-mx6 and the packaging system ended using the one
provided by MX6QDL SoC.

This issue was very hard to find out the cause and understand why it
was happening so I'd like to thank Richard Purdie for his help in
figuring it out.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>