kernel: add utility functions to compare thread priorities

Since lower-numbered thread priorities are higher, the code can be
misleading when comparing priorities, and often require the same type of
comments. Instead, use utility inline functions that does the
comparisons.

_is_prio_higher already existed, but add comparisons for "lower than",
"higher than or equal to" and "lower than or equal to".

Change-Id: I8b58fe9a3dd0eb70e224e970fe851a2575ad468b
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>