systemd-bootconf: Mark as machine specific

systemd-bootconf: Mark as machine specific

APPEND is usually attached to a machine. This patch avoids multiconfig
errors such as:

| NOTE: Direct dependencies are ['multiconfig:qt5022:/workdir/repo/poky/meta/recipes-core/glibc/glibc_2.29.bb:do_populate_sysroot', 'multiconfig:qt5022:virtual:native:/workdir/repo/poky/meta/recipes-devtools/pseudo/pseudo_git.bb:do_populate_sysroot', 'multiconfig:qt5022:/workdir/repo/poky/meta/recipes-devtools/quilt/quilt-native_0.65.bb:do_populate_sysroot', 'multiconfig:qt5022:/workdir/repo/poky/meta/recipes-devtools/gcc/gcc-cross_8.3.bb:do_populate_sysroot', 'multiconfig:qt5022:/workdir/repo/poky/meta/recipes-devtools/gcc/gcc-runtime_8.3.bb:do_populate_sysroot']
| NOTE: Installed into sysroot: []
| NOTE: Skipping as already exists in sysroot: ['glibc', 'pseudo-native', 'quilt-native', 'gcc-cross-x86_64', 'gcc-runtime', 'libgcc', 'linux-libc-headers', 'libtool-native', 'texinfo-dummy-native', 'libmpc-native', 'flex-native', 'automake-native', 'zlib-native', 'mpfr-native', 'gmp-native', 'binutils-cross-x86_64', 'xz-native', 'autoconf-native', 'gnu-config-native', 'gettext-minimal-native', 'm4-native']
| DEBUG: Python function extend_recipe_sysroot finished
| DEBUG: Executing shell function do_install
| install: cannot stat 'loader.conf': No such file or directory
| WARNING: exit code 1 from a shell command.
| ERROR: Function failed: do_install (log file is located at /workdir/build/tmp/work/bobcat-poky-linux/systemd-bootconf/1.00-r0/temp/log.do_install.737)
NOTE: recipe systemd-bootconf-1.00-r0: task do_install: Failed
ERROR: Task (multiconfig:qt5022:/workdir/repo/poky/meta/recipes-core/systemd/systemd-bootconf_1.00.bb:do_install) failed with exit code '1'

Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>