Allow devices to specify certain ro. props via TARGET_UNIFIED_DEVICE

This modifies buildinfo.sh to not set model, name, description, and
fingerprint in build.prop. These can then be set via custom init
lib.
This also fixes ota_from_target_files to function without these
props being preset.

Change-Id: I945b2c38e60b207d2d0f82b34f4f230ac21b0657
[HAX] Allow per-target dtbTool

 * Revert this when fixed

Change-Id: I97ecb0448ae7bd5859454be290c5dde6248b2859
gettop: fix symlink detection

Summary: We must use the -P flag for pwd to properly get $PWD from the environment *with* symlinks. Otherwise symlink
detection is broken and anything we do using $(gettop) at the top of a symlinked path won't work properly. I noticed this
issue while running on a system that has a SSD RAID symlinked to my android build home directory, which is ~/ssd_storage.
The top in this instance is a symlink, which means that gettop() returns my home path, or the path above the symlink
directory (but not the symlinked directory). This is a major issue in case developers choose to use $(gettop),
$ANDROID_BUILD_TOP, or $T (doesn't really matter either way).

Test Plan: Create a symlink and sync android source into the path. Open up the symlink, run envsetup.sh, and check
$(gettop) output.

While this isn't the best solution, it fixes an issue without creating new issues.

Change-Id: I65bc345753805e6161098d2af6e0bee0b07d37b1
Store the value from BOARD_CUSTOM_BOOTIMG_MK to a file

This is needed so it can be read in when re-signing a package to
use the proper boot image.

Print notice when using a custom bootimage makefile

Change-Id: I6baab9f8964ce5d4fa9487f2859af9c385d7b21b
roomservice: Add recursive dependencies

Dependency repositories can now have dependencies themselves

Change-Id: I33a28709170da52bc98f4a62387927e3a11b2450