Add directory support to nsjail genrule

Trusty build requires a lot of files. Rather than globbing all such
files (more than 10,000 files), allowing bind-mounting directories makes
nsjail much more efficient. With this change, directories can be
directly bind-mounted with dirgroup modules and dir_srcs property of
genrule module.

dirgroup module and dir_srcs property will be allowed only to Trusty
builds.

Bug: 358302178
Test: m lk.elf.x86_64 lk.elf.arm64
Change-Id: I5938a57b7fb65f8fecce4a8f40aa4aedbf991135