Revert "fdtdec: Use CONFIG_IS_ENABLED in board_fdt_blob_setup()"

On Rockchip platforms we need this area of code in TPL, but there is no
TPL_SEPARATE_BSS symbol.

This reverts commit 0a2aaab0.

Reported-by: Jesper Schmitz Mouridsen <jesper@schmitz.computer>
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>