stlink-common: Update STM32L0 and STM32L1 loader ABI

This patch invert source and destination registers in the stm32l0 and stm32l1
loaders, so that it follows the same ABI as other stm32 loaders.

Doing that, the run_flash_loader() function can be simplified a little.

Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>