ARC: [cmdline] uboot cmdline handling rework

* Moved cmdline copy from asm to "C" - allows for more robust checking
  of pointer validity etc.

* Remove the Kconfig option to do so, base it on a runtime value passed
  by u-boot

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --cc arch/arc/Kconfig
Simple merge

diff --cc arch/arc/kernel/head.S
Simple merge

diff --cc arch/arc/kernel/setup.c
Simple merge