Add flash partition capsule support

[helloxiling]

This PR introduces the flash partition capsule flash_partition.rs, which provides the syscall driver for user space applications to perform read, write, and erase operations on data of arbitrary length from the underlying flash storage. 

The detailed changes include: 

1. Code reorganization: creating a separate package named flash_driver to encapsulate the relevant flash driver files. 
2. Definition on the generic interface for reading, writing and erasing the arbitrary length of data on flash storage in flash_driver/hil.rs.
3. Implementation of the flash storage logical driver flash_storage_to_page.rs that maps read, write, and erase operations of arbitrary length into page-based operations. This driver is essential for enabling the flash partition capsule. 
4. Addition of in-kernel integration testing flash_storage_test.rs for the flash storage logical driver. 
5. Bug fix in flash_ctrl driver: ensuring interrupt clearance occurs before the callback, as the callback may initiate another flash operation. 
6. Instantiate one instance of flash partition in board.rs as a placeholder for user mode testing.
7. Minor update: changing page_size from 1024 to 256 in flash_ctrl driver and emulated flash_ctrl to reflect the most common NOR flash configuration.

The user mode testing on flash partition will be added in separate PR.