image.bbclass: Add package-management image feature to validitems

image.bbclass: Add package-management image feature to validitems

Some images dont use core-image bbclass and add this feature directly

Fixes parsing errors like

/builds1/angstrom/cache/prserv.sqlite3, IP: 127.0.0.1, PORT: 34734, PID:
24551 Parsing recipes...ERROR: Unable to parse
/builds1/angstrom/sources/meta-kde/recipes-images/angstrom-kde-desktop-imag
e.bb: Exited with "1" ERROR: 'package-management' in IMAGE_FEATURES is
not a
valid image feature. Valid features: dbg-pkgs debug-tweaks dev-pkgs
doc-pkgs ptest-pkgs read-only-rootfs splash staticdev-pkgs ERROR:
'package-management' in IMAGE_FEATURES is not a valid image feature.
Valid
features: dbg-pkgs debug-tweaks dev-pkgs doc-pkgs ptest-pkgs
read-only-rootfs splash staticdev-pkgs ERROR: Command execution failed:
Exited with 1

Signed-off-by: Khem Raj <raj.khem@gmail.com>