arm: keep all sections in ELF file

Current LDS files /DISCARD/ a lot of sections when linking ELF
files, causing diagnostic tools such as readelf or objdump to
produce partial output. Keep all section at link stage, filter
only at objcopy time so that .bin remains minimal.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>