linker: prefer board specific linker file

First look for a custom linker file defined by a config variable, if
not defined, look in the board directory. Finally look in the SoC
directory.

This adds flexibility and enhances modularity allowing a board to define
the linker the script instead of the SoC specific one.

Change-Id: Id44aa7d0e93d97234163ec858e2cfefe09768a08
Signed-off-by: Anas Nashif <anas.nashif@intel.com>