cortexm_common/ldscript: re-use _rom_offset variable name
cortexm_common/ldscript: re-use _rom_offset variable name

Inspired by kaspar030 version to removing the new _boot_offset variable.

https://github.com/kaspar030/RIOT/blob/cbf324a66d0c925dd02eca290e17c350945e97b2/cpu/cortexm_common/ldscripts/cortexm.ld

BytesGalore/RIOT,kbumsik/RIOT,kaspar030/RIOT,kYc0o/RIOT,biboc/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,lazytech-org/RIOT,smlng/RIOT,x3ro/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,cladmi/RIOT,A-Paul/RIOT,BytesGalore/RIOT,A-Paul/RIOT,gebart/RIOT,yogo1212/RIOT,OTAkeys/RIOT,mtausig/RIOT,OlegHahm/RIOT,mtausig/RIOT,basilfx/RIOT,OTAkeys/RIOT,toonst/RIOT,aeneby/RIOT,cladmi/RIOT,kYc0o/RIOT,x3ro/RIOT,BytesGalore/RIOT,josephnoir/RIOT,aeneby/RIOT,miri64/RIOT,kYc0o/RIOT,toonst/RIOT,OTAkeys/RIOT,A-Paul/RIOT,ant9000/RIOT,mfrey/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,gebart/RIOT,OlegHahm/RIOT,mfrey/RIOT,aeneby/RIOT,avmelnikoff/RIOT,kaspar030/RIOT,smlng/RIOT,cladmi/RIOT,yogo1212/RIOT,smlng/RIOT,basilfx/RIOT,RIOT-OS/RIOT,basilfx/RIOT,ant9000/RIOT,toonst/RIOT,cladmi/RIOT,yogo1212/RIOT,biboc/RIOT,authmillenon/RIOT,kaspar030/RIOT,biboc/RIOT,jasonatran/RIOT,mfrey/RIOT,miri64/RIOT,BytesGalore/RIOT,BytesGalore/RIOT,OlegHahm/RIOT,x3ro/RIOT,smlng/RIOT,biboc/RIOT,mtausig/RIOT,lazytech-org/RIOT,yogo1212/RIOT,josephnoir/RIOT,toonst/RIOT,ant9000/RIOT,cladmi/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,rfuentess/RIOT,kaspar030/RIOT,OlegHahm/RIOT,josephnoir/RIOT,jasonatran/RIOT,gebart/RIOT,lazytech-org/RIOT,smlng/RIOT,lazytech-org/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,x3ro/RIOT,authmillenon/RIOT,rfuentess/RIOT,gebart/RIOT,avmelnikoff/RIOT,josephnoir/RIOT,aeneby/RIOT,authmillenon/RIOT,rfuentess/RIOT,rfuentess/RIOT,ant9000/RIOT,yogo1212/RIOT,lazytech-org/RIOT,kaspar030/RIOT,x3ro/RIOT,mfrey/RIOT,yogo1212/RIOT,jasonatran/RIOT,OlegHahm/RIOT,mtausig/RIOT,mtausig/RIOT,kbumsik/RIOT,rfuentess/RIOT,basilfx/RIOT,mfrey/RIOT,avmelnikoff/RIOT,kbumsik/RIOT,aeneby/RIOT,A-Paul/RIOT,basilfx/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,miri64/RIOT,gebart/RIOT,toonst/RIOT,miri64/RIOT,kbumsik/RIOT,ant9000/RIOT,biboc/RIOT,kYc0o/RIOT,A-Paul/RIOT