arm: mvebu: Add runtime detection of UART (xmodem) boot-mode

This patch adds runtime detection of the Marvell UART boot-mode (xmodem
protocol). If this boot-mode is detected, SPL will return to the
BootROM to continue the UART booting.

With this patch its now possible, to generate a U-Boot image that
can be booted either from the strapped boot-device (e.g. SPI NOR, MMC,
etc) or via the xmodem protocol from the UART. In the UART case,
the kwboot tool will dynamically insert the UART boot-device type
into the image. And also patch the load address in the header, so
that the mkimage header will be skipped (as its not expected by the
Marvell BootROM).

This simplifies the development for Armada XP / 38x based boards.
As no special images need to be generated by selecting the
MVEBU_BOOTROM_UARTBOOT Kconfig option.

Since the Kconfig option MVEBU_BOOTROM_UARTBOOT is not needed any
more, its now completely removed.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <dirk.eibach@gdsys.cc>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Kevin Smith <kevin.smith@elecsyscorp.com>