meta-agl-bsp: Add CONFIG_LSM workaround for 5.1+ kernels

meta-agl-bsp: Add CONFIG_LSM workaround for 5.1+ kernels

Add explicit clearing of CONFIG_LSM before running "make oldconfig"
in the do_configure_append added by linux-agl.inc. This ensures that
the derived value of CONFIG_LSM will reflect configuration changes
done by the merged AGL provided fragments, i.e. SMACK being enabled.
Without it, kernel recipes that are not based on linux-yocto.bbclass
or have their own fragment handling done via prepend will end up with
an incorrect value of CONFIG_LSM which effectively disables SMACK.
Examples include the linux-imx or linux-fslc kernels used by NXP.

Bug-AGL: SPEC-2932

Change-Id: Id99906e3de75128ebc54866d43d89d3a24bb1cf2
Signed-off-by: Scott Murray <scott.murray@konsulko.com>