meta-tegra-support: add Mender workaround for kernel recipe

meta-mender-core adds a bbappend to all `linux-%` kernel
recipes that conditionally adds a config fragment to enable
EFI stub support. We don't need that support in linux-tegra,
however, and the FILESEXTRAPATHS change made in that bbappend
modifies the do_kernel_metadata task signature (per the
file-cksums setting in linux-yocto.bbclass).

So to allow us to share kernel artifacts between Mender and
non-Mender, add a bbappend when meta-mender-core is in the
layer set that undoes the changes made by their bbappend.

Signed-off-by: Matt Madison <matt@madison.systems>