imx: replace CONFIG_SECURE_BOOT with CONFIG_IMX_HAB

CONFIG_SECURE_BOOT is too generic and forbids to use it for cross
architecture purposes. If Secure Boot is required for imx, this means to
enable and use the HAB processor in the soc.

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/doc/imx/habv4/guides/encrypted_boot.txt b/doc/imx/habv4/guides/encrypted_boot.txt
index c59d204..e2b4357 100644
--- a/doc/imx/habv4/guides/encrypted_boot.txt
+++ b/doc/imx/habv4/guides/encrypted_boot.txt
@@ -16,7 +16,7 @@
 the dek_blob cmd enabled. The image used for DEK blob generation
 needs to have the following configurations enabled in Kconfig:
 
-CONFIG_SECURE_BOOT=y
+CONFIG_IMX_HAB=y
 CONFIG_CMD_DEKBLOB=y
 
 Note: The encrypted boot feature is only supported by HABv4 or