usttrace: Use /bin/sh instead of /bin/bash for busybox compatibility

The busybox posix like shell does not understand the "function"
directive nor does it understand the syntax for redirecting a file via
a shell expanded variable with $(<$pidfilepath).  Busybox also does
not typically provide a link to /bin/bash since busybox does not
provide bash.

It is possible to work around all these limitations in order to allow
user space tracing to work properly in a busybox based environment
with several syntax changes to the usttrace script.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>