Add an example under examples/storage/generic_partition_bdl that
partitions a raw block device at run time using only the Block Device
Layer (BDL) interface.
The example obtains a whole-disk BDL (SPI flash data partition by
default, or an SD/eMMC card via menuconfig), writes an MBR partition
table onto it with the esp_ext_part_tables managed component (fetched
via idf_component.yml), then creates a generic-partition BDL for each
MBR entry with esp_blockdev_generic_partition_get() and mounts FATFS on
the FAT slice and LittleFS on the LittleFS slice.
