arm: stm32mp1: force boot_device variable for invalid TAMP register value

arm: stm32mp1: force boot_device variable for invalid TAMP register value

When the TAMP register 20 have an invalid value (0x0 for example after
TAMPER error) the "boot_device" U-Boot env variable have no value and
no error is displayed in U-Boot log.

The STM32MP boot command bootcmd_stm32mp failed with strange trace:
  "Boot over !"

and the next command in bootcmd_stm32mp failed with few indication:
  if test ${boot_device} = serial || test ${boot_device} = usb;
	then stm32prog ${boot_device} ${boot_instance};

As it is difficult to investigate, the current patch avoids this issue:
- change the debug message to error: "unexpected boot mode" is displayed
- display trace "Boot over invalid!" in bootcmd_stm32mp
- execute "run distro_bootcmd" to try all the possible target

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>