xkeyboard-config: Bind square brackets for N900 us keymap

Seems there are issues with the keymaps we load in the initramfs using loadkmap, the keyboard is completely messed up.

It's not related to the changes to the us keymap in this MR because I've tested it on a previous installation on the device.
After a clean install through pmbootstrap, the keymap for the console broke.

The problems seems related to either loadkeys -b rx51_us.map > rx51_us.bmap or loadkmap < rx51_us.bmap

After some investigation I've found the problem:

• Alpine recently updated kdb to 2.2.0 from 2.0.4 (was very old version from 2007)
• kdb changed the binary format of the key value from unsigned short (2 bytes) to int (4 bytes)
  before: https://github.com/legionus/kbd/blob/8cc0ce8423973675871d3ca80603d174d3db6c2e/src/libkeymap/dump.c#L75-L78
  after: https://github.com/legionus/kbd/blob/master/src/libkeymap/dump.c#L75-L77
• busybox still reads it as unsigned short
  see https://github.com/brgl/busybox/blob/master/console-tools/loadkmap.c#L39

what to do?