Check out our u-boot port. It can be loaded instead of the linux kernel and chainloaded from the Samsung's SBL bootloader
You can edit the [include/configs/omap4_tuna] and comment the CONFIG_SPL_BUILD line and flash the u-boot.bin instead of the kernel.
Then, you can put the android boot.img to /system/boot/vmlinux.img
You can put the custom image in the u-boot mkimage or the android image format to /sdcard/boot/vmlinux.uimg and the boot script to /sdcard/boot/boot.scr.uimg, and you can boot that image by holding the volume down button.
And if you really wonder about the CONFIG_SPL_BUILD. Just stop reading here.
Yes, you can enable it and replace the Samsung's SBL with u-boot. Be careful. It only works with old xloader's (PRIMEKK14 and older). Display does not work, so you'll have to rebuild the kernel and disable omap framebuffer bootloader init check. If you brick the device, use "OMAPFlash_tuna.zip" to recover.
Apple Macbook Pro Retina 2012, Sony Xperia Z
Me on github
Me on gitorious
uboot for Samsung Galaxy S2 Samsung Galaxy Nexus
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