package/linux: don't enforce check for DTS when not building

Currently, this is triggering the error message:
    make randconfig
    make source

Limit the checks that enforce a DTS is set and at most one DTB is
appended to when we are actually building, like is done for the
configuration-file variables.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>