[WCNCR00152226] misc: Add build support for Linux backports-3.14-1

[Description]
Fix compile errors for Linux 2.6.32 + backports-3.14-1 due to:
- non-static inline functions
- function skb_reset_mac_len is not available before linux-3.0

Add a KAL function kal_skb_reset_mac_len to wrap implementations. Modify
Makefile.x86 and export $CFG_CFG80211_VERSION only when
$BACKPORTED_KERNEL_VERSION is non-empty.

Tested ok by
1. linux-2.6.32 (i386) + backports-3.14-1
2. build 12 variants ok:
    make -f Makefile.x86 \
       LINUX_SRC={3.10.23/arm, 3.18.42/i386, 4.4.23/amd64, 3.13.11/amd64} \
       hif={sdio, pcie, usb}

Change-Id: Ic1455a484164f93b57f05693921c03a98c23d758
Feature: misc
Signed-off-by: George Kuo <george.kuo@mediatek.com>
CR-Id: WCNCR00152226
5 files changed