bootserver: update only tftpboot dirs which are older than their images

One can use --force to update all dirs, regardless of their last
modification times.

(cherry picked from commit 2bb57dbd27f0466cd5338c53832640611d0b7df7)
bootserver: prune all orphan boot directories with --prune flag

(cherry picked from commit 90d4da7e234669a9e47905a5690521f222dfb749)
bootserver: rename update-tftpboot => copy-tftpboot

(cherry picked from commit 1f5d80c414b4c8b8ea3e085d98d5396e12837a8f)
bootserver: fix command line flag handling

(cherry picked from commit 0fe283184b18a83029529217b47b6e9a5d530ddd)
bootserver: add missing variable declaration

(cherry picked from commit e65cd9186ddb02582868e1be2a7ae5cf5fc40342)
bootserver: fix image filepath templating

(cherry picked from commit 0cf58eceb663deed859bde30fc1874b56bce5b4f)
bootserver: make tftpboot update handle symlink mtimes too

(cherry picked from commit ac282c01c0705e90197d73b9c8f18ca1ffaddb24)
bootserver: treat non-existing image boot dirs as really old

(cherry picked from commit 53e294c950ce0ff4e2ae8fc690c2295636a8aa04)