RDKCOM-2585: RDKBDEV-731: drop unnecessary local buffers
RDKCOM-2585: RDKBDEV-731: drop unnecessary local buffers

Reason for change:
drop unnecessary local buffers in lm_wrapper_get_arp_entries()
Also cleanup _get_shell_output() local function and make it static.
Test Procedure: Sanity.
Risks: None.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com>

(cherry picked from commit 8b4b29530f6e1257eed0d87b9f149a583e4e2554)
Change-Id: Ie783c498b768f018fd2bb134153a61fead2bec04

rdkcmf/rdkb-CcspLMLite,rdkcmf/rdkb-CcspLMLite,rdkcmf/rdkb-CcspLMLite