libpayload: Enable building libpayload with march=i586

Add a Kconfig value to enable building libpayload with the 586 compiler.
Update the cross compiler script to add the Kconfig value name that is
used when libpayload builds.

The Quark SOC does not support some of the instructions generated with
the 686 compiler (e.g. CMOV).  Success occurs when
payloads/libpayload/build/config.h indicates that
CONFIG_LP_USE_MARCH_586=1.

TEST=Build and run on Galileo Gen2.

Change-Id: I04907e9a38ee139bae2e8b227821f54614707c25
Signed-off-by: Lee Leahy <Leroy.P.Leahy@intel.com>
Reviewed-on: https://review.coreboot.org/20322
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/Kconfig

@@ -157,6 +157,14 @@ config BASE_ADDRESS
 
 endmenu
 
+config USE_MARCH_586
+	bool "Use march=586 qualifier to build"
+	default n
+	depends on ARCH_X86
+	help
+	  Allow a platform or processor to select to be compiled using
+	  the '-march=i586' option instead of the typical '-march=i686'
+
 menu "Standard Libraries"
 
 config LIBC

util/xcompile/xcompile

@@ -239,7 +239,7 @@ EOF
 if [ "${TARCH}" = "x86_64" ] || [ "${TARCH}" = "x86_32" ]; then
 cat <<EOF
 
-ifeq (\$(CONFIG_USE_MARCH_586),y)
+ifneq (\$(CONFIG_USE_MARCH_586)\$(CONFIG_LP_USE_MARCH_586),)
 	GCC_CFLAGS_${TARCH} += -march=i586
 else
 	GCC_CFLAGS_${TARCH} += -march=i686