Support ChromeOS partition table and kernel partition (MR 2163)

Stock bootloader on these devices boots kernel (it is intended to boot
kernel, but i place secondary bootloader there) from special ChromeOS
kernel partition on special GPT which is created with cgpt utility.
This MR adds initial support for it introducing new deviceinfo options:

- cgpt_kpart - path to file to be flashed to ChromeOS partition;
- cgpt_kpart_start - offset from the start in sectors;
- cgpt_kpart_size - partition size in sectors.

For example:

deviceinfo_cgpt_kpart="/usr/share/u-boot/google-peach-pit/u-boot-dtb.img.kpart"
deviceinfo_cgpt_kpart_start="8192"
deviceinfo_cgpt_kpart_size="32768"

cgpt requires start and size values of partition, so these values
are calculated for each partition.

Reserved size and on-device installer are not yet supported.

Reference: https://archlinuxarm.org/platforms/armv7/samsung/samsung-chromebook

pmb/helpers/frontend.py

@@ -260,6 +260,10 @@ def install(args):
         if args.filesystem:
             raise ValueError("--on-device-installer cannot be combined with"
                              " --filesystem")
+
+        if args.deviceinfo["cgpt_kpart"]:
+            raise ValueError("--on-device-installer cannot be used with"
+                             " ChromeOS devices")
     else:
         if args.ondev_cp:
             raise ValueError("--cp can only be combined with --ondev")