The process for building recovery is pretty straight forward once you have the Cyanogen source code and managed to extract the boot image from your target device. I built a basic CM recovery based on the boot. The next option was to flash the recovery using the SP Flash Tool. So far it appears that Mediatek LK based bootloaders do not support the OEM unlock command, hence it is not possile to flash the recovery using fastboot through an unlocked bootloader. For Mediatek based devices, the fasboot mode is handled by the Little Kernel. The bootloader unlocking process is described in official google documentationit can be roughly summarized to. Technically this means there are well defined specification and tools for easily unlocking the bootloader. However, Android has always been developer friendly i.
Android developer documentation explains verified boot process in greater detail. Over the years, the Android boot process has become more complex with cryptographic means to establish trust for bootloader and maintain a chain of trust for subsequent components involved in the boot process. The locked bootloader ensures that the OS cannot be flashed using software means, at least without first unlocking the bootloader.
Almost all Android based device comes with a locked bootloader. Once unlocked, it is possible to perform flashing usi ng the fastboot protocol while the device is in fastboot mode. For example, Nexus based devices support the flashboot mode with unlockable bootloader. The purpose of unlocking the bootloader is to allow the bootloader software to flash update specific partitions in the eMMC.
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