arm: make __image_copy_{start, end} compiler-generated

This change is only done where needed: some linker
scripts may contain __image_copy_{start,end} yet
remain unchanged.

Also, __image_copy_end needs its own section; putting
it in relocation sections changes their flags and makes
relocation break.

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>