images: add HABv4 support for i.MX6 ...

This patch adds high assurance boot support (HABv4) image generation to
barebox, currently tested on i.MX6 only.

In order to build a signed barebox image, add a new image target to
images/Makefile.imx as illustrated in the diff below:

- - - a/images/Makefile.imx
+ + + b/images/Makefile.imx
@@ -163,10 +163,14 @@ image-$(CONFIG_MACH_SABRELITE) += barebox-freescale-imx6dl-sabrelite.img
 pblx-$(CONFIG_MACH_SABRESD) += start_imx6q_sabresd
 CFG_start_imx6q_sabresd.pblx.imximg = $(board)/freescale-mx6-sabresd/flash-header-mx6-sabresd.imxcfg
 FILE_barebox-freescale-imx6q-sabresd.img = start_imx6q_sabresd.pblx.imximg
 image-$(CONFIG_MACH_SABRESD) += barebox-freescale-imx6q-sabresd.img

+CSF_start_imx6q_sabresd.pblx.imximg = $(havb4_imx6csf)
+FILE_barebox-freescale-imx6q-sabresd-signed.img = start_imx6q_sabresd.pblx.imximg.signed
+image-$(CONFIG_MACH_SABRESD) += barebox-freescale-imx6q-sabresd-signed.img
+

Here the default i.MX6 CSF file $(havb4_imx6csf) is used, it's generated during
build on from the template "scripts/habv4/habv4-imx6.csf.in". You can configure
the paths to the SRK table and certificates via: System Type -> i.MX specific
settings -> HABv4 support.

The proprietary tool "cst" by Freescale tool is expected in the PATH.

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