android-tools: add new package

This package allows to build the fastboot and adb host utilities,
which can be used to interact with target devices implementing one of
these protocols.

The work behind the host utilities was funded by ECA Group
<http://www.ecagroup.com

>. ECA Group is the copyright owner of the
contributed code.

The package also allows to build fastboot, adb and adbd daemon for the
target.

Regarding adbd, the target is required to have the FunctionFS USB Gadget
configuration. Then the following commands enable the use of adb:
 # modprobe g_ffs idVendor=0x18d1 idProduct=0x4e42 \
   iSerialNumber="buildroot"
 # mkdir -p /dev/usb-ffs/adb
 # mount -t functionfs adb /dev/usb-ffs/adb -o uid=2000,gid=2000
 # adbd &

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Tested-by: Julien Corjon <corjon.j@ecagroup.com>
[Thomas:
 - update on top of master.
 - fix Config.in.host prompt, it should have been "host android-tools"
   and not just "android-tools".]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>