arm: separate .data and .text sections of EFI binaries

EFI binaries should not contain sections that are both writable and
executable. Separate the RX .text section from the RW .data section.

We currently don't created relocation sections (.rel.*) for our EFI
binaries. Anyway these would have to be converted to PE/COFF relocations.
Enumerate them under DISCARD and add a comment.

Correct the characteristics of the sections.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>