scripts: use 'userprogs' to build programs for target ...

Use 'userprogs' syntax to build standalone programs for the target
architecture (i.e. the same architecture as the barebox).

This changes the compiler flags passed to the target programs.

Previously, it used $(KBUILD_CFLAGS), which contains the same compiler
flags as used for the barebox space.

Going forward, it will use $(KBUILD_USERCFLAGS), which contains smaller
set of compiler flags because there is no need to add low-level options
for userspace builds.

The shortlog will be annotated with [U].

  CC [U]  scripts/bareboxenv-target
  CC [U]  scripts/kernel-install-target
  CC [U]  scripts/bareboxcrc32-target
  CC [U]  scripts/bareboximd-target

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>