install: add a --sector-size flag (MR 2391)

This can be used when building images for generic device targets that
support devices with different sector size requirements.

For example, trailblazer prebuilts are currently expected to be flashed
to a USB drive where a 4096 sector size would be unsuitable since the
bootloader wouldn't detect it. But when building for a Qualcomm phone,
one would use --split and --sector-size to build the root and boot
partitions with a 4k sector size which is appropriate to the UFS
storage.

This flag could also be used by BPO to build both variants.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/types.py

@@ -152,6 +152,7 @@ class PmbArgs(Namespace):
     rsync: bool
     scripts: str
     second_storage: str
+    sector_size: int | None
     selected_providers: dict[str, str]
     sparse: bool
     split: bool