Draft: new commits for adding support for SECO-NE SAN boards

Hello @tobias.poganiuch ,

this MR is a continuation for https://git.seco.com/edgehog/layers/seco/meta-seco-imx/-/merge_requests/223 (as part of https://secogroup.atlassian.net/browse/YT-91 task on JIRA).

In particular, when I worked on one of the open problems discovered earlier, I found and addressed two new problems:

1. A non-working fstab is generated for Edgehog because a wrong "ondisk" argument is set in WKS file, so /dev/mmcblkp1 is used for /boot instead of /dev/mmcblk0p1.

2. A first stage bootloader for SAN boards (FNG Boot) fails to start U-Boot when a meta-seco-edgehog-embedded partition scheme is used for WIC. It tries to boot FNG Boot way and does not start U-Boot. So, I added boot.cfg and U-Boot binary to the /boot partition.

Best regards, Mikhail Vanyulin.