Setting up a new build environment for Openwrt, I discovered that the compilation of aprx fails due to a missing file.
netax25.c:27:10: fatal error: netax25/ax25.h: No such file or directory
The quick and dirty solution is to add a directory nextax25 and place the missing files into that directory:
For target BRCM47XX
For target ip140xx
The files you need are ax25.h and sockaddr.h, which can find on several other places, after you have a complete compilation of openwrt without aprx.