habv4: habv4_get_status(): fix warning with no-op static inline function ...

This patch fixes the following warning:

  CC      arch/arm/boards/foo/board.o
  In file included from arch/arm/boards/foo/board.c:22:0:
  include/habv4.h:24:19: warning: function declaration isn't a prototype [-Wstrict-prototypes]
   static inline int habv4_get_status()
                      ^

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

habv4: add High Assurance Boot v4 ...

This patch adds the code to check the HAB ROM for failures during development.
Add a cal to "habv4_get_status();" to your board file, to get the current
system state from the ROM.

_NOTE_: On i.MX6 this has to happen before barebox starts the MMU, because the
        HAB ROM vector table is placed at 0x94, which is not accessible after the
	MMU has setup the zero page.

This patch contains code ported from u-boot patches [1][2] by Shaojun Wang [3]
which were found in the "Mx28 Secure Boot" and "Mx6 HAB (High Assurance Boot)"
thread on the freescale community forum [4][5].

[1] https://community.freescale.com/servlet/JiveServlet/download/370047-269174/0001-enable-mx28-u-boot-hab.patch.txt.zip
[1] https://community.freescale.com/servlet/JiveServlet/download/96451-11-266175/0001-u-boot-enable-mx6-hab.patch.zip
[2] https://community.freescale.com/people/ShaojunWang
[3] https://community.freescale.com/thread/317254
[3] https://community.freescale.com/docs/DOC-96451

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>