profiles: Define a default for ACCEPT_LICENSE.

profiles: Define a default for ACCEPT_LICENSE.

In its 2019-02-10 meeting, the Gentoo Council has decided that the
default for ACCEPT_LICENSE should be changed to @FREE. We take that
opportunity to move the default to make.defaults in the base profile,
for the following reasons:
 - The setting depends on the @FREE license group, which itself is not
   defined in Portage, but in profiles/license_groups.
 - Setting the default in profiles will allow to perform the switchover
   at a precise time, independent of Portage release and stabilisation
   schedule.
 - Profiles are a better location to define the default distro policy.
 - GLEP 23 says so.

Bug: https://bugs.gentoo.org/677800
Signed-off-by: Ulrich Müller <ulm@gentoo.org>