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main/postmarketos-base: add sysctl config to disable rp_filter on ipv4

Administrator requested to merge no_rp_filter into master

MMS support (via mmsd-tng) involves sending/receiving network requests/responses over the wwan interface. If it's ipv4-only and the device is connected to some other ipv4 network on another iface (like wifi), this can cause the rp_filter to reject responses on wwan iface because it incorrectly thinks they are martian packets.

This does theoretically disable some "security" feature in the kernel, but it's worth noting that:

  1. rp_filter isn't implemented at all in the kernel for ipv6

  2. other distros (mobian, pureos at least) are also disabling rp_filter

  3. this seems to be a relatively common problem with folks using mms on pmOS, since many carriers' data networks are ipv4-only

also see: kop316/mmsd!55 (b22c253f)

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