x86: Change how selection of ROMs works

Most x86 boards build a u-boot.rom which is programmed into SPI flash. But
this is not unique to x86. For example some rockchip boards can also boot
from SPI flash.

Also, at least on x86, binary blobs are sadly quite common. It is not
possible to build a functional image without them, and U-Boot needs to
know this at build time.

Introduce a new CONFIG_HAS_ROM option which selects whether u-boot.rom is
built and a new CONFIG_ROM_NEEDS_BLOBS option to indicate whether binary
blobs are also needed. If they are not needed, it is safe to build the ROM
always. Otherwise we still require the BUILD_ROM environment variable.

For now this affects only x86, but future patches will enable this for
rockchip too.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ff4f06e..01ffaea 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -360,6 +360,8 @@
 	bool "Add an Firmware Support Package binary"
 	depends on !EFI
 	select USE_HOB
+	select HAS_ROM
+	select ROM_NEEDS_BLOBS
 	help
 	  Select this option to add an Firmware Support Package binary to
 	  the resulting U-Boot image. It is a binary blob which U-Boot uses
@@ -519,6 +521,8 @@
 
 config HAVE_MRC
 	bool "Add a System Agent binary"
+	select HAS_ROM
+	select ROM_NEEDS_BLOBS
 	depends on !HAVE_FSP
 	help
 	  Select this option to add a System Agent binary to