Allow an alternate startup file independent of bootloader builds
Allow an alternate startup file independent of bootloader builds

In support of bootloader builds the nuttx_v3 builds previously had a
facility to set the start up files: Non Bootloader STM32 builds uses
the common vectors facility in NuttX. The bootloaders use a fixed
minimum set of vectors.

Since other architectures my need to include a start up file or
set of files, this PR allow an alternate startup file to be
selected independent of if this is bootloader build.

acfloria/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,jlecoeur/Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,acfloria/Firmware,Aerotenna/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,jlecoeur/Firmware,krbeverx/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,dagar/Firmware,Aerotenna/Firmware,PX4/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,krbeverx/Firmware,PX4/Firmware,mcgill-robotics/Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware