uboot-config: fix devtool modify with kernel-fitimage

uboot-config: fix devtool modify with kernel-fitimage

How to reproduce:
- UBOOT_CONFIG must be used. With UBOOT_MACHINE it works fine.
  A simple example based on oe-core is to modify the
  beaglebone-yocto.conf file like this:
  -UBOOT_MACHINE = "am335x_evm_defconfig"
  +UBOOT_CONFIG = "foo"
  +UBOOT_CONFIG[foo] = "am335x_evm_defconfig"
- A build configuration which inherits the kernel-fitimage.bbclass is
  needed. For example:
  MACHINE = "beaglebone-yocto"
  KERNEL_IMAGETYPE = "Image"
  KERNEL_IMAGETYPES += " fitImage "
  KERNEL_CLASSES = " kernel-fitimage "

devtool modify linux-yocto
devtool build linux-yocto
...
| cp: cannot stat '.../linux-yocto-6.6.21+git/am335x_evm_defconfig/.config':
  No such file or directory
| WARNING: .../linux-yocto/6.6.21+git/temp/run.do_configure.2081673:172 exit 1
  from 'cp .../linux-yocto-6.6.21+git/am335x_evm_defconfig/.config
  .../build/workspace/sources/linux-yocto/.config.baseline'

The reason for this problem is that the uboot-config.bbclass sets the
variable KCONFIG_CONFIG_ROOTDIR to a path that makes sense for u-boot,
but not for other recipes. However, the kernel-fitimage.bbclasse, for
example, inherits the uboot-config.bbclass, which brings the
u-boot-specific path into the kernel build context.

This change removes the uboot-specific KCONFIG_CONFIG_ROOTDIR path from
recipes other than u-boot itself.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>