L476RG won't boot with GCC_ARM on master

### Description
On mbed master, after merge 17q2 workshop branch for bootloader support, NUCLEO_L476RG target will not boot with GCC_ARM toolchain.

## Bug

**Target**
NUCLEO_LL476RG

**Toolchain:**
GCC_ARM

**Steps to reproduce**
any test in mbed2 or mbed5

