MPAS fails to build in a CESM tag and MPAS regression test fail

[jtruesdal]

In the top-level MPAS Makefile the directory to the MPAS source wasn't set properly to account for the difference in the CAM/CESM component directory structure. The MPAS make variable MPAS_SRC_ROOT is set correctly only for standalone CAM directory structure.
The fix is a one line change to cam/bld/configure.
-MPAS_SRC_ROOT := $cam_root/src/dynamics/mpas
+MPAS_SRC_ROOT := $cam_dir/src/dynamics/mpas

There is an old default file that is causing a failure in ERC_D_Ln9.mpasa480z32_mpasa480.FHS94.cheyenne_intel.cam-outfrq3s
-atm/cam/inic/mpas/mpasa480_L32_v6.1.grid_c190924.nc

• atm/cam/inic/mpas/mpasa480_L32_notopo_grid_c201125.nc