install/partition: allow specifying a different boot partition start (!1856)

This adds a new deviceinfo parameter, 'boot_part_start' which accepts an
int and indicates the number of sectors from the start of the drive to
place the boot partition.

The librem5 devkit (and actual phone) u-boot has grown beyond the 2048
sector space previously before the boot partition, so this is necessary in
order to boot pmos on this device.

pmb/install/_install.py

@@ -340,7 +340,8 @@ def embed_firmware(args):
                                "{}".format("/usr/share/" + binary))
         # Insure that embedding the firmware will not overrun the
         # first partition
-        max_size = (2048 * 512) - (offset * step)
+        boot_part_start = args.deviceinfo["boot_part_start"] or "2048"
+        max_size = (int(boot_part_start) * 512) - (offset * step)
         binary_size = os.path.getsize(binary_path)
         if binary_size > max_size:
             raise RuntimeError("The firmware is too big to embed in the "

pmb/install/partition.py

@@ -74,9 +74,11 @@ def partition(args, size_boot):
     # sometimes "fails to inform the kernel". In case it really failed with
     # partitioning, the follow-up mounting/formatting will not work, so it
     # will stop there (see #463).
+    boot_part_start = args.deviceinfo["boot_part_start"] or "2048"
+
     commands = [
         ["mktable", "msdos"],
-        ["mkpart", "primary", filesystem, "2048s", mb_boot],
+        ["mkpart", "primary", filesystem, boot_part_start + 's', mb_boot],
         ["mkpart", "primary", mb_boot, "100%"],
         ["set", "1", "boot", "on"]
     ]