envsetup: Add pushboot function

This function lets you push a file from your current OUT dir
for your current device. It root/remount the device, push the
file, and reboots it. Example: pushboot system/app/SystemUI.apk

Change-Id: I8a404323a4091e8502fbdd24198c66f1ac598192
envsetup: set OUT_DIR to an absolute path always

OUT_DIR was set to $(TOPDIR)out previously,
but $(TOPDIR) was null, so it was a relative path.
This broke releasetools, inline kernel building, etc
since they require absolute paths.
Fix it so that it is set to $(shell readlink -f .)/out
if $(TOPDIR) is null.

Also remove hacks which checked if (OUT_DIR) was out
and changed it to $(ANDROID_BUILD_TOP)/out to workaround
the aforementioned problem.

Change-Id: I459a3b1325a1bbea0565cd73f6acf160d4ed9b39

Conflicts:
	core/base_rules.mk
	core/tasks/kernel.mk

Change-Id: I52a7a6024f0f2b708c230e53c0152ed138726cc1