Hi, Jean
If I understand correctly, the terms of "the level of sophistication in the i.MX6 ROM bootloader is capable of loading & executing the Linux kernel directly" you mentioned is the function of direct jump to the entry point from ROM. This is what we done for DSM(deep sleep mode) mode. The pre-condition is that you put linux kernel suspend into DRAM, and then power off the i.MX6 but keeping DRAM powered on and in self-refresh, then when you power up the i.MX6 again, the boot ROM detect that there is valid entry point, it will jump to the entry point and resume the previou linux kernel before suspend. So, technically, it is a new low power mode called low power state retension mode, it need board design support, such as keeping DRAM powered on when i.MX6 is powered off etc..
Sorry that I can NOT tell you that which member of i.MX family has such feature, as far as I know, only since i.MX6, we linux BSP support suspend with ARM core power off and need boot ROM to jump to kernel directly when resume. But I think for other i.MX chips, you also can make this feature work, you can make the uboot jump to the previous kernel resume point instead of ROM, the only latency compared to i.MX6 is some more instructions of ROM code and a couple of instruction of uboot need to be run, you should be able to hit the 1 second target as well. This is my opinion, hope it is helpful for you.
