[PATCH] imx: hab: add documentation about the required keys/certs

[Claudius Heine <ch@denx.de>]

For CST to find the certificates and keys for signing, some keys and
certs need to be copied into the u-boot build directory.

Signed-off-by: Claudius Heine <ch@denx.de>
---
 doc/imx/habv4/guides/mx8m_spl_secure_boot.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/doc/imx/habv4/guides/mx8m_spl_secure_boot.txt b/doc/imx/habv4/guides/mx8m_spl_secure_boot.txt
index ce1de659d8..42214df21a 100644
--- a/doc/imx/habv4/guides/mx8m_spl_secure_boot.txt
+++ b/doc/imx/habv4/guides/mx8m_spl_secure_boot.txt
@@ -144,6 +144,22 @@ The signing is activated by wrapping SPL and fitImage sections into nxp-imx8mcst
 etype, which is done automatically in arch/arm/dts/imx8m{m,n,p,q}-u-boot.dtsi
 in case CONFIG_IMX_HAB Kconfig symbol is enabled.
 
+Per default the HAB keys and certificates need to be located in the build
+directory, this means copying the following files from the HAB keys directory
+flat (e.g. removing the `keys` and `cert` subdirectory) into the u-boot build
+directory for the CST Code Signing Tool to locate them:
+
+- `crts/SRK_1_2_3_4_table.bin`
+- `crts/CSF1_1_sha256_4096_65537_v3_usr_crt.pem`
+- `keys/CSF1_1_sha256_4096_65537_v3_usr_key.pem`
+- `crts/IMG1_1_sha256_4096_65537_v3_usr_crt.pem`
+- `keys/IMG1_1_sha256_4096_65537_v3_usr_key.pem`
+- `keys/key_pass.txt`
+
+The paths to the SRK table and the certificates can be modified via changes to
+the nxp_imx8mcst device tree node, however the other files are required by the
+CST tools as well, and will be searched for in relation to them.
+
 Build of flash.bin target then produces a signed flash.bin automatically.
 
 1.4 Closing the device
-- 
2.42.0