install: add a --sector-size flag

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.

cc @TravMurav