Simplify the imx6-device-mkspec

Simplify the imx6-device-mkspec

This change is possible due to commit 18654058
As it's now possible to use configure's -sysroot without
gcc's --sysroot, it's possible to use configure's pkg-config setup
(which comes with its -sysroot). This gives the possibility to
simplify the mkspec and be in line with the other device-mkspecs

Change-Id: I0f34ec2c5c7b06334ad3c1605001c0803272b027
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>