docs/getting_started/build-options.rst clearly says ENABLE_PIE is
currently only supported in BL31, but in fact, it has a stronger
limitation:

  Defining ENABLE_PIE may corrupt BL1 and BL2. So, ENABLE_PIE is
  supported only for platforms where BL31 is the only image built
  in the TF-A tree.

Currently, ENABLE_PIE is enabled by two platforms,
plat/arm/common/arm_common.mk and ti/k3/common/plat_common.mk,
both of which enable ENABLE_PIE together with RESET_TO_BL31.

For platforms with the full boot sequence, ENABLE_PIE may break earlier
BL stages. For example, if I build PLAT=qemu with ENABLE_PIE=1, it
fails in BL1.

When ENABLE_PIE is enabled, PIE options are added to TF_CFLAGS and
TF_LDFLAGS, so all BL images are affected. It is problematic because
currently only the BL31 linker script handles it. Even if BL1/BL2
works, the image size would increase needlessly, at least.

Pass the PIE options only to BL images that support it.

Change-Id: I550e95148aa3c63571c8ad2081082c554a848f57
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>