Swap uvisor and uvisor-lib

Main changes

• ARMmbed/uvisor-lib is not a submodule of ARMmbed/uvisor any more.
• All APIs are now put in a centralized folder — api.
• The uVisor binary is now released as a static library which includes the compiled uVisor APIs as well.
• As a result, uVisor is now de-coupled from the host OS. uvisor-lib is the glue layer needed to integrate the uVisor in mbed OS and yotta. Read the updated documentation for more details.

API changes

• Removed vIRQ_SetVectorX. Use vIRQ_SetVector, instead.

Bug fixes

• Check for an uninitialized debug box.
• Prevent null handlers in the debug box driver.
• Distinguish between checks on public flash and physical flash.
• Avoid dependency on reset value of MPU RNR register.
• Restore priority bits when writing to the NVIC IP register.