Yes, I wrote one bootloader for a SoC, and read through several other bootloaders to be able to make modifications to them. I've also reverse engineered several binary only bootloaders for undocumented MCUs to be able to write sane Linux based flashing tools for them.
Kinda know what bootloader is. And, while I don't care about the naming, I know several people who would vehemently disagree with you that grub is not a bootloader. They'd say that only grub is a bootloader, and anything else running prior to that is platform firmware.
Cortex-A is not typically used in MCUs, so I don't see what you mean by "The MCU is the first thing to boot".
Kinda know what bootloader is. And, while I don't care about the naming, I know several people who would vehemently disagree with you that grub is not a bootloader. They'd say that only grub is a bootloader, and anything else running prior to that is platform firmware.
Cortex-A is not typically used in MCUs, so I don't see what you mean by "The MCU is the first thing to boot".