kleaf: set MO for kernel_module and drop VPATH.

6.13 requires MO to be set to put external kernel module artifacts
in a separate directory. Set it here.

Drop the VPATH trick for 6.13 because

(1) it is unofficial
(2) it breaks the build
(3) it was a workaround because separate build directory for
    external modules was not supported.

Bug: 402231876
Change-Id: I7701019a17788db0226e0a8ee7143b428a21121e