I hope this helps.
First I have the latest Debian OS from 10/2018
and I have the Element14 WIFI cape working with the industrial BeagleBone Black
It took a lot of poking around.
But God smiled upon the struggle and I found BB-GATEWAY-WL1837-00A0.dtbo in the /lib/firmware directory.
Edit the /boot/uEnv.txt with these modifications.
###Additional custom capes
uboot_overlay_addr4=/lib/firmware/BB-GATEWAY-WL1837-00A0.dtbo
Then modify this sectoin
###Disable auto loading of virtual capes (emmc/video/wireless/adc)
disable_uboot_overlay_emmc=1
disable_uboot_overlay_video=1
disable_uboot_overlay_audio=1
#disable_uboot_overlay_wireless=1
disable_uboot_overlay_adc=1
Comment out
enable_uboot_cape_universal=1
Add this line.
cape_enable=bone_capemgr.enable_partno=BB-BONE-WL183
power down
PLUG your wifi cape in and and power up.
start connmanctl and do what the readme says to do.
enable your wifi, scan your wifi etc…
You should be running and not tearing your hair out.
Have a great day