Arm: Fix error message printing in board makefile

Remove an incorrect tabulation in front of an $(error) function call
outside of a recipe, which caused the following text to be displayed:

  plat/arm/board/common/board_common.mk:36: *** recipe commences before first target.  Stop.

instead of:

  plat/arm/board/common/board_common.mk:36: *** "Unsupported ARM_ROTPK_LOCATION value".  Stop.

Change-Id: I8592948e7de8ab0c4abbc56eb65a53eb1875a83c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
1 file changed