kernel: only strip proc for small flash devices

kernel: only strip proc for small flash devices

Currently, you are not able to get statistics about IPv4 and IPv6
usage. This information can be collected via the snmp and snmp6.
However, in the current state this interface is disabled as you can
read in the "902-debloat_proc.patch":
 "Strip non-essential /proc functionality to reduce code size"

Tools like netstat use the snmp/6 interface to collect interface
statistics. Some prometheus exporters also mention this:
- prometheus-collectors/netstat.lua
- prometheus-collectors/snmp6 (still a PR)
- collectd/snmp6 (still a PR)

PRs:
- https://github.com/collectd/collectd/pull/3789
- https://github.com/openwrt/packages/pull/14158

Instead of enabling it as default for all devices we condition it
 default y if SMALL_FLASH

A test shows it needs around 16 kiB.

Signed-off-by: Nick Hainke <vincent@systemli.org>
[fixed whitespace issue]
Signed-off-by: Petr Štetiar <ynezz@true.cz>