debug: coredump: support dumping stack top only

Add Kconfig option to dump only a portion of stack from the
current stack pointer to the stack end. This is enough to
let gdb reconstruct the stack trace and can significantly
reduce the dump size. This is crucial if the core dump needs
to be sent over radio.

Additionally, add another option to set the limit for the
dumped stack portion.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>