Support for u-boot in android boot image

U-boot FIT image - image format, described as a device tree markup, may contain any files, including but not limited - kernels, initramfs, device trees, u-boot boot scripts. Also contains any number of configurations. File override feature may be used to pack android boot image with u-boot instead of linux kernel and FIT image instead of initramfs.

New device-specific control options for images generation:

• deviceinfo_generate_uboot_fit_images: set "true" to generate U-boot FIT images, one image per *.its file
• deviceinfo_bootimg_override_payload: file name to be used instead of linux kernel file in android boot image
• deviceinfo_bootimg_override_initramfs: file name to be used instead of initramfs file in android boot image
• deviceinfo_bootimg_override_payload_compression: stock bootloader usually expect gzip payload, and fail, if it's not.
• deviceinfo_bootimg_override_payload_append_dtb: stock bootloader usually expect dtb appended to the payload, and fail, if it's not. A dumb dtb may be created to save space, with only memory and chosen nodes.

If a device have u-boot support, an android boot image may be generated with u-boot, as a second stage bootloader instead of linux kernel, and u-boot's FIT image instead of initramfs. To generate such an image, you should:

• add u-boot support for your device in Alpine u-boot package
• add deviceinfo_bootimg_override_payload option in device package with "u-boot.bin" value
• add deviceinfo_bootimg_override_initramfs option (.itb file), and FIT source file(.its) in device package This allows using u-boot without storage support, so that stock bootloader runs u-boot, and u-boot runs FIT image, places instead of initramfs. Note, your u-boot port should know, where initramfs is loaded by stock bootloader.