AndroidKernel: Use TARGET_KERNEL_CROSS_COMPILE_PREFIX for both 32/64 bit

TARGET_TOOLS_PREFIX is a new variable introduced in build environment
which keeps the location of the toolchain from the android source, which
was working for 64bit but giving unwind warnings for 32 bit targets.

[    7.353691] VFS: Mounted root (ext4 filesystem) readonly on device 253:0.
[    7.363324] Freeing unused kernel memory: 2048K
[    7.375778] unwind: Unknown symbol address c010130c
[    7.375802] unwind: Index not found c010130c
[    7.379466] unwind: Unknown symbol address c010130c
[    7.383992] unwind: Index not found c010130c
[    7.388578] unwind: Unknown symbol address c010130c
[    7.393096] unwind: Index not found c010130c
[    7.397690] unwind: Unknown symbol address c010130c
[    7.402227] unwind: Index not found c010130c
[    7.406806] unwind: Unknown symbol address c010130c
[    7.411331] unwind: Index not found c010130c 

To solve this let's make the toolchain path prefix change in the target
BoardConfig file as per architecture(32/64) basis and avoid making change
in kernel.

Change-Id: I6ca9642aeabbba1b892e169816f3d6d260bd50d5
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>