package/elf2flt: add support for XTENSA_[PN]DIFF relocations

package/elf2flt: add support for XTENSA_[PN]DIFF relocations

Xtensa have added new relocation types R_XTENSA_[NP]DIFF{8,16,32} with
the same properties as the existing types R_XTENSA_DIFF{8,16,32}.
Add them to the list of ignored relocation types.

This fixes the following error when invoking elf2flt on xtensa binaries
built with the recent binutils:

  ERROR: reloc type R_XTENSA_PDIFF32 unsupported in this context

Reported-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>