Add option to set shell prompt string from menuconfig and migrate
boards globally.

The migration is done as follows:
- Boards that explicitly and unconditionally set CONFIG_SYS_PROMPT had the
entry moved to their defconfig files.
- Boards that defined some kind of #ifdef logic which selects the
CONFIG_SYS_PROMPT (for example qemu-mips) got an #undef CONFIG_SYS_PROMPT
right before the #ifdef logic and were left alone.
- This change forces CONFIG_SYS_PROMPT to be a per board decision, and thus
CONFIG_SYS_PROMPT was removed from all _common.h and _common.h
files. This results in a streamlined default value across platforms, and
includes the following files: spear-common, sunxi-common, mv-common,
ti_armv7_common, tegra-common, at91-sama5_common, and zynq-common.
- Boards that relied on _common.h values of CONFIG_SYS_PROMPT were
not updated in their respective defconfig files under the assumption that
since they did not explicitly define a value, they're fine with whatever
the default is.
- On the other hand, boards that relied on a value defined in some
_common.h file such as woodburn_common, rpi-common,
bur_am335x_common, ls2085a_common, siemens_am33x_common, and
omap3_evm_common, had their values moved to the respective defconfig files.
- The define V_PROMPT was removed, since it is not used anywhere except for
assigning a value for CONFIG_SYS_PROMPT.

Cc: Tom Rini
Cc: Masahiro Yamada
Cc: Stefano Babic
Cc: Igor Grinberg
Signed-off-by: Nikita Kiryanov
[trini: Add spring, sniper, smartweb to conversion]
Signed-off-by: Tom Rini