Makefile: fix host CPP pollution in cmd_gen_envp

Makefile: fix host CPP pollution in cmd_gen_envp

Commit 2956a84ba701 ("Makefile: don't use CFLAGS for environment text
file") switched cmd_gen_envp to use $(cpp_flags) so that KCPPFLAGS would
be honored when preprocessing the environment text file. However,
cpp_flags also includes $(PLATFORM_CPPFLAGS), which carries
architecture-specific flags such as -march=armv8-a+crc set by
arch/arm/Makefile for CONFIG_ARM64_CRC32=y targets. These flags are
appropriate for cross-compiled object files but cause the host CPP
invocation to fail on x86 build machines:

  cc1: error: bad value ('armv8-a+crc') for '-march=' switch

KBUILD_CPPFLAGS already accumulates the user-supplied KCPPFLAGS via
"KBUILD_CPPFLAGS += $(KCPPFLAGS)", providing the behaviour 2956a84
intended without pulling in target architecture flags. Use it directly.

Fixes: 2956a84ba701 ("Makefile: don't use CFLAGS for environment text file")
Signed-off-by: Levi Shafter <levi.shafter@elder-tomes.com>
Reviewed-by: Simon Glass <sjg@chromium.org>