default or selected kernelversion does not have config value CONFIG_OMAP1_MBOX.
Result is shown for kernelversion 6.5.1

OMAP1 Mailbox framework support

modulename: mailbox_omap1.ko

configname: CONFIG_OMAP1_MBOX

Linux Kernel Configuration
└─>Device Drivers
└─>Mailbox Hardware Support
└─>OMAP1 Mailbox framework support
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Mailbox implementation for OMAP chips with hardware for
interprocessor communication involving DSP in OMAP1. Say Y here
if you want to use OMAP1 Mailbox framework support.

source code: