arm: generalize lib/bss.c into lib/sections.c

File arch/arm/lib/bss.c was initially defined for BSS only,
but is now going to also contain definitions for other
section-boundary-related symbols, so rename it for better
accuracy.

Also, remove useless 'used' attributes.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>
Tested-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>