[U-Boot] Can I load uEnv.txt conditionally, based on the boot source?

[Brian Smucker]

Hello,

I was wondering if there is a way to tell what the boot source is?

My am335x SOC can boot to the mmc or the flash, based on the status of a 
digital line. I would like to load the uEnv.txt, only if booting from SD 
card (mmc).  I notice at least in some codebases that the automatic 
loading of uEnv.txt is based on whether the SD card is available, not 
whether we are currently booting from the SD card.

Is there a way to sense which we are booting from?

Thanks

Brian