build: entirely use hybris-boot to generate bootimage.

[peat-psuwit]

This commit remove remaining Ubuntu Touch codes regarding ramdisk and
boot image handling, and entirely offload this work to hybris-boot
package.

With this commit, both bootimage and systemimage can be built
successfully for Fairphone 2.

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Three things to bear in mind:

1. The code for bootimage is entirely removed out of this file.
2. Calling make bootimage will produce $(OUT)/hybris-boot.img instead of $(OUT)/boot.img.
3. This commit doesn't handle recoveryimage. So, calling make recoveryimage will still produce Android-style recovery boot image as before. (BTW is it working in Halium at all?)

This is quite an intrusive change. Test thoroughly on a few different devices before merging this in.