rootfs_deb/package_deb.bbclass: move func from rootfs_deb to package_deb
package_deb.bbclass:
1. Added new function package_update_index_deb() to generate package
index files.
2. Added new function package_install_internal_deb() to install a list
deb packages to a specified root directory, with the specified package
architecutre information.
rootfs_deb.bbclass:
Used the above new functions to install the rootfs.
