skeleton: add /dev/fd, /dev/std{in, out, err} symlinks for static /dev on readonly...

Some applications, e.g. bashs process subsitution feature, rely on the
convention of `/dev/fd` being a symbolic link to `/proc/self/fd`.

When a static /dev is used on a readonly rootfs then the runtime ln
invocations in the inittab will fail, so we need to add the symlinks at
build time.  Makedevs doesn't support creating symlinks, so instead add the
symlinks to the default skeleton.

For non-static /dev setups, the kernel will mount devtmpfs which shadows the
/dev of the rootfs, but then the runtime ln invocations in inittab will
create the symlinks.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

diff --git a/system/skeleton/dev/fd b/system/skeleton/dev/fd
new file mode 120000 (symlink)
index 0000000..10e8ab7
--- /dev/null
+++ b/system/skeleton/dev/fd
@@ -0,0 +1 @@
+../proc/self/fd
\ No newline at end of file

diff --git a/system/skeleton/dev/stderr b/system/skeleton/dev/stderr
new file mode 120000 (symlink)
index 0000000..c67586e
--- /dev/null
+++ b/system/skeleton/dev/stderr
@@ -0,0 +1 @@
+../proc/self/fd/2
\ No newline at end of file

diff --git a/system/skeleton/dev/stdin b/system/skeleton/dev/stdin
new file mode 120000 (symlink)
index 0000000..2825dc0
--- /dev/null
+++ b/system/skeleton/dev/stdin
@@ -0,0 +1 @@
+../proc/self/fd/0
\ No newline at end of file

diff --git a/system/skeleton/dev/stdout b/system/skeleton/dev/stdout
new file mode 120000 (symlink)
index 0000000..35e0f62
--- /dev/null
+++ b/system/skeleton/dev/stdout
@@ -0,0 +1 @@
+../proc/self/fd/1
\ No newline at end of file