bash: Enable system-wide .bashrc file, source /etc/shinit

This enables a system-wide .bashrc file (/etc/bash.bashrc), as well as
some other options related to startup files, and sources /etc/shinit for
interactive shells.

Fixes https://bugs.openwrt.org/index.php?do=details&task_id=3019.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

utils/bash/files/etc/bash.bashrc

@@ -0,0 +1,6 @@
+# System-wide .bashrc file
+
+# Continue if running interactively
+[[ $- == *i* ]] || return 0
+
+[ \! -s /etc/shinit ] || . /etc/shinit