This patch removes the variable U_BOOT from the swupdate-config.bbclass. It also adds the suffix `SWUPDATE` to the variable BOOTLOADER.
If a custom u-boot is used in concunction with swupdate it is recommened to set `U_BOOT_CONFIG_PACKAGE` to '1'. This now adds a dependency from SWupdate to u-boot-{MACHINE}-config for installing fw_env.config.
Changes in V2: - fix typo in commit message - use variable in kas/opt/*.yml
Changes in V3: - remove variable U_BOOT - split into 2 commits