[efi] Treat writable sections as data sections

[efi] Treat writable sections as data sections

Hybrid bzImage and UEFI binaries (such as wimboot) may include 16-bit
executable code that is opaque data from the perspective of a UEFI PE
binary, as described in wimboot commit fe456ca ("[efi] Use separate
.text and .data PE sections").

The ELF section will be marked as containing both executable code and
writable data.  Choose to treat such a section as a data section
rather than a code section, since that matches the expected semantics
for ELF files that we expect to process.

Signed-off-by: Michael Brown <mcb30@ipxe.org>