configs: k2g_evm: Make findfdt command populate fdtfile variable

On all other platforms the command 'findfdt' populates the variable
'fdtfile', but on K2G we only populate 'name_fdt'. The generic boot
and automation scripts fail when 'findfdt' is not populated, fix
this for K2G.

Signed-off-by: Andrew F. Davis <afd@ti.com>