Build: merge build macros between FIP_ and FWU_FIP_

Build: merge build macros between FIP_ and FWU_FIP_

The build system supports generating two FIP images, fip and fwu_fip.
Accordingly, we have similar build macros.

   FIP_ADD_PAYLOAD   <-->  FWU_FIP_ADD_PAYLOAD
   CERT_ADD_CMD_OPT  <-->  FWU_CERT_ADD_CMD_OPT
   FIP_ADD_IMG       <-->  FWU_FIP_ADD_IMG

The duplicated code increases the maintenance burden.  Also, the build
rule of BL2U looks clumsy - we want to call MAKE_BL to compile it from
source files, but we want to put it in fwu_fip.  We can not do it in a
single macro call since the current MAKE_BL does not support fwu_fip.

To refactor those in a clean way is to support one more argument to
specify the FIP prefix.  If it is empty, the images are targeted to
fip, whereas if the argument is "FWU_", targeted to fwu_fip.

The build macros prefixed with FWU_ go away.

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