shell: optionally configure priority of the shell thread

Currently, in Zephyr the shell thread is the lowest priority i.e.
K_LOWEST_APPLICATION_THREAD_PRIO. Due to this all the other threads can
preempt the shell thread. Proposed solution is to add Kconfig which
gives the flexibility to change the priority of the shell thread.

This is now implemented using a new Kconfig variable
SHELL_THREAD_PRIORITY_OVERRIDE. By setting this option
SHELL_THREAD_PRIORITY can be set.

Signed-off-by: Uday Ammu <udaykiran@google.com>