riscv: merge two if-blocks for KBUILD_IMAGE

riscv: merge two if-blocks for KBUILD_IMAGE

In arch/riscv/Makefile, KBUILD_IMAGE is assigned in two separate
if-blocks.

When CONFIG_XIP_KERNEL is disabled, the decision made by the first
if-block is overwritten by the second one, which is redundant and
unreadable.

Merge the two if-blocks.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240323113500.1249272-1-masahiroy@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>