title: Bootloader: verify user application before handover
body: <p>I am writing a second stage bootloader for SAMD21, which hands over to a userland application after hardware initialization.</p>
  
  <p>How do I verify that the jump address with the application reset handler (eg <code>0x2000</code>) contains actually valid code?</p>
  
  <p>The goal is to abort the jump if there is no valid userland handler to avoid undefined behaviour.</p>