So far, board.d files were having execute bit set and contained a
shebang. However, they are just sourced in board_detect, with an
apparantly unnecessary check for execute permission beforehand.

Replace this check by one for existance and make the board.d files
"normal" files, as would be expected in /etc anyway.

Note:

This removes an apparantly unused '#!/bin/sh /etc/rc.common' in
target/linux/bcm47xx/base-files/etc/board.d/01_network

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>