First of all, I forgot to thank everyone who has been helping me so far.
So after banging my head against the wall last night, this is what I can say.
Going into the GRUB command line, I listed the partitions with "ls" to see if I can manually set root and load the kernel and initrd.
Grub lists the desired partition as (hd0,gpt5) and I'm able to view its contents from within GRUB console, but when loading TC it still doesn't see /dev/mmcblk
I've tried different options, including booting with "waitusb=10", setting UUID in boot options, doing "insmod" of part_gpt and gzio in GRUB (basically copying everything from Ubuntu GRUB menuentries) and even compiling the kernel for the first time (yay!) setting all MMC related options to Y, but still nothing.
I also wonder why "mmc_block" module is not loaded by default in TC while "mmc_core" is. Does anyone have a clue why?
So I assume there is still something in kernel options I might need to configure.
I did a diff between the .config used to compile Ubuntu and TC kernels. Now, I know it's a lot of text, but if anyone here has any experience compiling kernels for eMMC devices, pointing me in the right direction would be much appreciated