Add bootloaderd support to NUCLEO_F446RE target

[pavels]

Description

This simply adds support for bootloader for STM32F446RE target (NUCLEO_446RE)

Status

READY

Migrations

NO

[0xc0170]

Hi @pavels , can you please sign https://os.mbed.com/contributor_agreement/ ?

I restarted travis CI, we are currently looking at circle CI that does not have proper configuration.

cc @bcostm @adustm @LMESTM @jeromecoutant

[pavels]

Hi, i have signed the agreement, but have a different username on mbed site - @pavels6

[LMESTM]

LGTM
@pavels can you please just confirm how you've tested it ?

[pavels]

@LMESTM i have NUCLEO-F446RE board and i am currently running mbed application with modified version of cvra bootloader ( https://github.com/cvra/can-bootloader ) - the bootloader BIN image is padded to 48k so it occupies first 3 16bytes sectors in flash

[0xc0170]

@pavels Can you rebase please to the latest master (will fix some of Ci failures)

[mbed-ci]

Build : SUCCESS

Build number : 158
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5228/

Triggering tests

/test mbed-os

[mbed-ci]

Test : SUCCESS

Build number : 64
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5228/64