Changing the MTU for wwan affects the "rx_urb_size" value. When
"rx_urb_size" (effectively an MRU) is too low, the host rejects
packets from the radio, causing a packet loss, connection issues,
increasing interface error counters, and (potentially) high load on
the host CPU to the point when the host effectively stops
responding to user input (reproduced on MTCDT-L4E1 with Atmel).

Decouple MTU and "MRU" by setting "rx_usb_size" to a relatively
high value that is different from MTU.

NOTE: The target kernel in 6.3.x is linux-at91_5.4.199 as of the
time of writing.

Assumptions:

- Cellular networks never send packets bigger than 1500 bytes;
- the radio may send a packet bigger than the MTU size
  reported in wds-get-current-settings;
- mLinux and mPower do not use packet aggreggation (like qmap),
  hence no need in MTU/MRU sizes bigger than 1500.

nftables is only for the future versions of the firmware, the 6.3.x
branch shall keep using iptables.

Revert "Merge branch 'md/nftables' into '6'"

This reverts commit 8b53cf652131da6537871492dcb7a3cb055a72cb, reversing
changes made to b9d90d071db617bd845293612f41fcb1059f5911.

nftables: Enables nftables, disabled iptables kernel options

See merge request !37

- marked `CONFIG_MMC_ATMELMCI` (the SD-card kernel module) as a built-in
  module - required to boot from rootfs on the SD card;
- maked `CONFIG_REGMAP_I2C` as a built-in module - automatically by menuconfig,
  may be unnecessary;
- maked `CONFIG_EXT4_FS` as a built-in module - to mount EXT4 rootfs;
- maked `CONFIG_JBD2` and `CONFIG_FS_MBCACHE` as built-ins - automatically by
  menuconfig, may be unnecessary;
- removed automatically by menuconfig: `CONFIG_GCC_PLUGIN_STRUCTLEAK_USER`,
  `CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF`,
  `CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL`.