dtc: Make -I dtb mode use fill_fullpaths()

At present -I dts and -I fs modes both use the fill_fullpaths() helper
function to fill in the fullpath and basenamelen fields of struct
node, which are useful in later parts of the code.  -I dtb mode,
however, fills these in itself.

This patch simplifies flattree.c by making -I dtb mode use
fill_fullpaths() like the others.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>