Add a new CMake function esp_partition_flash_binary() that provides a
unified API for registering partition data binaries to be flashed. It
replaces the direct esptool_py_flash_target calls scattered across
components (spiffs, fatfs, nvs_flash) with a single function that:
- Resolves partition offset from the partition table automatically
- Determines encryption requirements (auto-detect or ALWAYS_PLAINTEXT)
- Creates per-partition flash targets (e.g. idf.py <partition>-flash)
- Optionally includes the binary in `idf.py flash` via FLASH_IN_PROJECT
On the linux target, the function registers binaries for pre-loading
into the emulated flash. A build-time manifest (linux_flash_data.txt)
is generated via file(GENERATE), and partition_linux.c reads it at
runtime to copy each binary into the memory-mapped flash buffer at
the correct offset.
The partition_ops example is updated to use the new function and
includes a custom_partition with pre-built data to demonstrate the
full workflow, including on the linux target.
