Constant L4_FLASH_SR_PEMPTY 

pub(crate) const L4_FLASH_SR_PEMPTY: u32 = _; // 131_072u32

FLASH_SR.PEMPTY (bit 17) on the STM32L4.

Records whether user flash read as empty at the last reset. While it is set, the part boots its system ROM bootloader instead of the image, so it must be clear after programming an erased part; see clear_l4_pempty.