Xtensa port: Increased idle thread stack size to avoid stack overflow.

Xtensa port uses more stack than others. This was discussed with the team and
we agreed that this can be accepted for the first beta.
We will investigate this later to see how to avoid allocating coproc registers
for the system threads in order to reduce the stack overhead. However this
will not be before the port is considered stable.

Change-Id: Icd5b2b0ab68d0906b5408f35f081b100acabc010
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>