CONFIG_ATH79_MACH_PB44 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Atheros PB44 reference board

modulename: mach-pb44.ko

configname: CONFIG_ATH79_MACH_PB44

Linux Kernel Configuration
└─>Atheros AR71XX/AR724X/AR913X machine selection
└─>Atheros PB44 reference board
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
Say 'Y' here if you want your kernel to support the
Atheros PB44 reference board.

source code:
arch/mips/ath79/mach-pb44.c

depends
CONFIG_ATH79