Fixed tftpboot symlink check

As it is possible to use autofs to mount the medias we need to set
symlinks to the autofs root folder. Now the condition checks if the
symlink is set within the tftpboot root, if it is everything is fine, if
it points outside of the root the installation stops with an error
message.
I don't see it working:

1st test: 

dirpath with symlink + last component non-symlink: 

does not work because the -L $MEDIA check fails

2nd test: 

stale symlink in path gives syntax error as readlink -e resolves to empty result, which gives: 

bash: conditional binary operator expected
bash: syntax error near `/srv/tftpboot/.*'


3rd test: symlink pointing to /home/srv/tftpboot/foo

does not work because your regex does not have an anchor.