kernel-fitimage.bbclass: introduce FIT_UBOOT_ENV

Introduce a new variable FIT_UBOOT_ENV, which allows to add a u-boot
script as a text file to the fitImage. Such a script can be sourced
from the u-boot shell, as documented here:
https://docs.u-boot.org/en/latest/usage/cmd/source.html#fit-image

The kernel-fitimage.bbclass also evaluates the existing UBOOT_ENV
variable and adds the corresponding script to the fitImage. However, the
UBOOT_ENV variable is also used to install the script as an old u-boot
image, usually named boot.scr into the /boot directory of the target
device. These are different use cases which should be handled
independently.

Appending the script to the u-boot recipe and then hand it over to the
kernel recipe via the staged /boot directory leads to complicated task
dependencies. Decoupling the two use cases will also allow to simplify
the implementation by dropping the evaluation of the UBOOT_ENV variable
in the kernel-fitimage.bbclass. But this commit is supposed to be
backward compatible.

(From OE-Core rev: 269605ed053fd8dc7bcbcc04a46c308188115f66)

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/kernel-fitimage.bbclass

@@ -616,6 +616,11 @@ fitimage_assemble() {
 			bbwarn "${STAGING_DIR_HOST}/boot/${UBOOT_ENV_BINARY} not found."
 		fi
 	fi
+	if [ -n "${FIT_UBOOT_ENV}" ]; then
+		cp ${UNPACKDIR}/${FIT_UBOOT_ENV} ${B}
+		bootscr_id="${FIT_UBOOT_ENV}"
+		fitimage_emit_section_boot_script $1 "$bootscr_id" ${FIT_UBOOT_ENV}
+	fi
 
 	#
 	# Step 4: Prepare a setup section. (For x86)

meta/conf/image-fitimage.conf

@@ -51,3 +51,10 @@ FIT_ADDRESS_CELLS ?= "1"
 # Then the images signature checking will not be mandatory and no error will be
 # raised in case of failure.
 # UBOOT_SIGN_IMG_KEYNAME = "dev2" # keys name in keydir (eg. "dev2.crt", "dev2.key")
+
+# Additional environment variables or a script which can be sourced by u-boot
+# can be packaged into the fitImage. An example is documented here:
+# https://docs.u-boot.org/en/latest/usage/cmd/source.html#fit-image
+# Machine configurations needing such a script file should include it in the
+# SRC_URI of the kernel recipe and set the FIT_UBOOT_ENV parameter.
+FIT_UBOOT_ENV ?= ""