Feature/basic flash serial

[elsalahy]

Summary:

Adds support to SPI and external flash with block APIs support and test bench.

Relates to #16 and #30

Changes:

• Added the BSP SPI support
• Added the MX25R16 official library with support to all commands
• Added basic app init example and a simple test app.

Notes for Reviewers:

None

Release Notes: (optional)

...