linker: prefer board specific linker file
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 <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

rsalveti/zephyr,rsalveti/zephyr,Vudentz/zephyr,punitvara/zephyr,holtmann/zephyr,finikorg/zephyr,32bitmicro/zephyr,fbsder/zephyr,32bitmicro/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,bigdinotech/zephyr,mirzak/zephyr-os,fbsder/zephyr,bboozzoo/zephyr,galak/zephyr,runchip/zephyr-cc3200,coldnew/zephyr-project-fork,pklazy/zephyr,zephyriot/zephyr,galak/zephyr,bigdinotech/zephyr,coldnew/zephyr-project-fork,pklazy/zephyr,32bitmicro/zephyr,erwango/zephyr,rsalveti/zephyr,holtmann/zephyr,jamesonwilliams/zephyr-kernel,zephyriot/zephyr,zephyriot/zephyr,pklazy/zephyr,nashif/zephyr,ldts/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,erwango/zephyr,punitvara/zephyr,erwango/zephyr,punitvara/zephyr,kraj/zephyr,mirzak/zephyr-os,galak/zephyr,sharronliu/zephyr,tidyjiang8/zephyr-doc,nashif/zephyr,finikorg/zephyr,holtmann/zephyr,tidyjiang8/zephyr-doc,mirzak/zephyr-os,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,mirzak/zephyr-os,mbolivar/zephyr,pklazy/zephyr,finikorg/zephyr,jamesonwilliams/zephyr-kernel,tidyjiang8/zephyr-doc,fractalclone/zephyr-riscv,tidyjiang8/zephyr-doc,bigdinotech/zephyr,fractalclone/zephyr-riscv,GiulianoFranchetto/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3220,Vudentz/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,32bitmicro/zephyr,fractalclone/zephyr-riscv,zephyriot/zephyr,galak/zephyr,runchip/zephyr-cc3200,fractalclone/zephyr-riscv,GiulianoFranchetto/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,punitvara/zephyr,holtmann/zephyr,mbolivar/zephyr,Vudentz/zephyr,runchip/zephyr-cc3200,fbsder/zephyr,sharronliu/zephyr,bboozzoo/zephyr,mbolivar/zephyr,kraj/zephyr,Vudentz/zephyr,runchip/zephyr-cc3200,kraj/zephyr,explora26/zephyr,punitvara/zephyr,bboozzoo/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,mirzak/zephyr-os,aceofall/zephyr-iotos,aceofall/zephyr-iotos,jamesonwilliams/zephyr-kernel,explora26/zephyr,aceofall/zephyr-iotos,nashif/zephyr,mbolivar/zephyr,coldnew/zephyr-project-fork,Vudentz/zephyr,coldnew/zephyr-project-fork,holtmann/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,erwango/zephyr,jamesonwilliams/zephyr-kernel,mbolivar/zephyr,jamesonwilliams/zephyr-kernel,aceofall/zephyr-iotos,coldnew/zephyr-project-fork,rsalveti/zephyr,pklazy/zephyr,ldts/zephyr,ldts/zephyr,ldts/zephyr,bboozzoo/zephyr,bigdinotech/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,kraj/zephyr,galak/zephyr,bboozzoo/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,fbsder/zephyr,erwango/zephyr,fbsder/zephyr,Vudentz/zephyr,rsalveti/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,32bitmicro/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3200,aceofall/zephyr-iotos,bigdinotech/zephyr