libebl: Use elf_getshdrstrndx in ebl_section_strip_p.

libebl: Use elf_getshdrstrndx in ebl_section_strip_p.

The ebl_section_strip_p function used the Ehdr e_shstrndx field
to get at the name of the (debug) sections. This is not correct
if there are more than SHN_LORESERVE sections. Use elf_getshdrstrndx
to get at the shstrtab section. And drop the Ehdr argument that isn't
necessary anymore.

Signed-off-by: Mark Wielaard <mark@klomp.org>