u-boot: ensure keys are generated before assembling U-Boot FIT image

Add the task dependency:

do_uboot_assemble_fitimage -> virtual/kernel:do_kernel_generate_rsa_keys

to ensure the kernel FIT image signing keys are available when creating
the U-Boot DTB. This is done only if the signing of the kernel FIT image
is enabled (UBOOT_SIGN_ENABLE="1").

The lack of the dependency causes build errors when executing a build
with no kernel FIT keys initially present in the keys directory. In such
cases one would see an output like this in the Bitbake logs:

Log data follows:
| DEBUG: Executing shell function do_uboot_assemble_fitimage
| Couldn't open RSA private key: '/workdir/build/keys/fit/dev.key': No such file or directory
| Failed to sign 'signature' signature node in 'conf-1' conf node
| FIT description: Kernel Image image with one or more FDT blobs
| ...

This issue was introduced by commit 259bfa86f384 where the dependency
between U-Boot and the kernel was removed (for good reasons). Before
that commit the dependency was set via DEPENDS so that, in terms of
tasks, one had:

u-boot:do_configure -> virtual/kernel:do_populate_sysroot

and the chain leading to the key generation was:

virtual/kernel:do_populate_sysroot -> virtual/kernel:do_install
virtual/kernel:do_install -> virtual/kernel:do_assemble_fitimage
virtual/kernel:do_assemble_fitimage -> virtual/kernel:do_kernel_generate_rsa_keys

With the removal of the first dependency, no more guarantees exist that
the keys would be present when assembling the U-Boot FIT image. That's
the situation we are solving with the present commit.

Fixes: 259bfa86f384 ("u-boot: kernel-fitimage: Fix dependency loop if UBOOT_SIGN_ENABLE and UBOOT_ENV enabled")
(From OE-Core rev: 036f20156b3c7d0a8b912e90aa29a9b986106d5a)

Signed-off-by: Rogerio Guerra Borin <rogerio.borin@toradex.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: 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/uboot-sign.bbclass

@@ -113,6 +113,8 @@ python() {
     sign = d.getVar('UBOOT_SIGN_ENABLE') == '1'
     if d.getVar('UBOOT_FITIMAGE_ENABLE') == '1' or sign:
         d.appendVar('DEPENDS', " u-boot-tools-native dtc-native")
+    if d.getVar('FIT_GENERATE_KEYS') == '1' and sign:
+        d.appendVarFlag('do_uboot_assemble_fitimage', 'depends', ' virtual/kernel:do_kernel_generate_rsa_keys')
 }
 
 concat_dtb() {