scripts: build-all: Build make targets sequentially.

Added omap2 to the list of defconfigs to scan.
Install headers and kernel modules to separate staging directories
by setting values for INSTALL_HDR_PATH and INSTALL_MOD_PATH.
Build make targets sequentially instead all on one invocation.

Signed-off-by: Andrew Walker <andreww@codeaurora.org>
Change-Id: I87dae363bd536808a74ff368b905d18ae11607b2