stmhal: Set entry point for ELF binary debugging

When loading the ELF binary to the board with a debugger, the debugger
needs to know at which point to start executing the code. Currently the
entry point defaults to the start of the .text section.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Merge pull request #922 from swegener/for-upstream

stmhal: Set entry point for ELF binary debugging