linux-yocto-custom: Unblock overrides of linux-yocto.inc

linux-yocto.inc defines some overridable variable with ?=
and we do the same in the atmel recipes, and since linux-yocto.inc
is included before those overrides the ones from linux-yocto.inc
takes precedence. Lets fix it by delaying the inclusion until
after we have weakly defined them ourselves. Another way to fix
it would be to use hard assignments but we then can not override
them further in our own bbappends

Signed-off-by: Khem Raj <raj.khem@gmail.com>