pmb.chroot.mount: create /dev/fd (MR 2170)

Specifically, this fixes issues with process substitution in bash in the
chroot.

For example, the following lines from a NetworkManager build script:

    ... |
        grep -Fx -f <(get_symbols_explicit) -v |
        grep -Fx -f <(get_symbols_nm)

fail with:
     ninja: job failed: /home/pmos/build/src/NetworkManager-1.35.92/tools/create-exports-NetworkManager.sh --called-from-build /home/pmos/build/src/NetworkManager-1.35.92
     grep: /dev/fd/63: No such file or directory
     grep: /dev/fd/63: No such file or directory

pmb/chroot/mount.py

@@ -72,6 +72,9 @@ def mount_dev_tmpfs(args, suffix="native"):
                                 "tmpfs", dev + "/shm"])
     create_device_nodes(args, suffix)
 
+    # Setup /dev/fd as a symlink
+    pmb.helpers.run.root(args, ["ln", "-sf", "/proc/self/fd", f"{dev}/"])
+
 
 def mount(args, suffix="native"):
     # Mount tmpfs as the chroot's /dev