To allow overriding of the boot stack size from the Mbed configuration
system, consistently use MBED_CONF_TARGET_BOOT_STACK_SIZE rather than
MBED_BOOT_STACK_SIZE.

Fixes #10319

### Description

The Mbed 2 release was broken for FF_LPC546XX as it would include
the entire mbed 2 build of the LPC546XX. This PR seperates these 2
builds completely by:

 * Removing non-shared extra labels from the MCU_LPC546XX target
 * Moving the target implementation to a shared target directory

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change