arm:manage versioning of u-boot and upgrade u-boot

Replaced the wget download of u-boot by a versioned git checkout
this allows us to better manage the u-boot and MLO version we ship
while still allowing us to build ofline.

This changes replaces the BASE_URL setting by U_BOOT_BIN_DIR and
also updates to a newer build of u-boot.
arm:refactor replace cmdline.txt by bootargs passed to the kernel.

Put the boot arguments in uEnv.txt and not in cmdline.txt to allow
a more dynamic configuration of the system. We now also pass the
u-boot board_name parameter to the kernel.
arm:determine board_id using bootargs.board_name

On startup determine the board_id based on the board name
passed from u-boot. This code also export "board" for use
by  userland using sysenv.

Change-Id: I1064a49497c82b06f50d98650132bc0a7f543568
arm:switch to dynamic configuration for the kernel.

During startup machine.board_id is now determined. The kernel can now
at runtime determine how to configure itself and does so.

Change-Id: I4f615af9bfa5add219e618b911a51af127591d6a
arm:replace ifdef's by runtime checks in the i2c driver.

Change-Id: I5cdff48d8be9bbe5a4734192b451dc45983fbef2
arm:make the kernel server and driver binaries identical.

Make the kernel server and driver binaries indentical for the different
ARM platforms. We no longer need to define the AM335X or DM37XX flags
during compilation. The remaining differences are all located in u-boot.

Change-Id: I14ac60837dd2d7f116bc5fa56d3989b48d7c4fea