Corrected main thread stack size, was accidently updated by removing condition
Corrected main thread stack size, was accidently updated by removing condition

Main thread stack size is reduced for TARGET_STM32F070RB and TARGET_STM32F072RB,
in recent update to mbed_rtx.h the conditional check for main thread size update
was removed.

mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed