Complete the new comtest_strings.c file.

Complete the new comtest_strings.c file.
Somewhat optimise the calling of vTaskSwitchContext() from interrupts in the new MicroBlaze port to ensure it does not get called multiple times when more than one peripheral is called from the same interrupt.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@1469 1d2547de-c912-0410-9cb9-b8ca96c0e9e2