ARM: make ARCH_MULTIPLATFORM user-visible

Some options like CONFIG_DEBUG_UNCOMPRESS and CONFIG_CMDLINE_FORCE are
fundamentally incompatible with portable kernels but are currently allowed
in all configurations. Other options like XIP_KERNEL are essentially
useless after the completion of the multiplatform conversion.

Repurpose the existing CONFIG_ARCH_MULTIPLATFORM option to decide
whether the resulting kernel image is meant to be portable or not,
and using this to guard all of the known incompatible options.

This is similar to how the RISC-V kernel handles the CONFIG_NONPORTABLE
option (with the opposite polarity).

A few references to CONFIG_ARCH_MULTIPLATFORM were left behind by
earlier clanups and have to be removed now up.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/Kconfig

@@ -320,7 +320,19 @@ config ARCH_MMAP_RND_BITS_MAX
 	default 16
 
 config ARCH_MULTIPLATFORM
-	def_bool MMU && !(ARCH_FOOTBRIDGE || ARCH_RPC || ARCH_SA1100)
+	bool "Require kernel to be portable to multiple machines" if EXPERT
+	depends on MMU && !(ARCH_FOOTBRIDGE || ARCH_RPC || ARCH_SA1100)
+	default y
+	help
+	  In general, all Arm machines can be supported in a single
+	  kernel image, covering either Armv4/v5 or Armv6/v7.
+
+	  However, some configuration options require hardcoding machine
+	  specific physical addresses or enable errata workarounds that may
+	  break other machines.
+
+	  Selecting N here allows using those options, including
+	  DEBUG_UNCOMPRESS, XIP_KERNEL and ZBOOT_ROM. If unsure, say Y.
 
 menu "Platform selection"
 	depends on MMU
@@ -1609,6 +1621,7 @@ config CMDLINE_EXTEND
 
 config CMDLINE_FORCE
 	bool "Always use the default kernel command string"
+	depends on !ARCH_MULTIPLATFORM
 	help
 	  Always use the default kernel command string, even if the boot
 	  loader passes other arguments to the kernel.

arch/arm/Kconfig.debug

@@ -1904,6 +1904,7 @@ config DEBUG_UART_8250_PALMCHIP
 
 config DEBUG_UNCOMPRESS
 	bool "Enable decompressor debugging via DEBUG_LL output"
+	depends on !ARCH_MULTIPLATFORM
 	depends on !(ARCH_FOOTBRIDGE || ARCH_RPC || ARCH_SA1100)
 	depends on DEBUG_LL && !DEBUG_OMAP2PLUS_UART && \
 		     (!DEBUG_TEGRA_UART || !ZBOOT_ROM) && \

arch/arm/kernel/devtree.c

@@ -194,14 +194,12 @@ const struct machine_desc * __init setup_machine_fdt(void *dt_virt)
 {
 	const struct machine_desc *mdesc, *mdesc_best = NULL;
 
-#if defined(CONFIG_ARCH_MULTIPLATFORM) || defined(CONFIG_ARM_SINGLE_ARMV7M)
 	DT_MACHINE_START(GENERIC_DT, "Generic DT based system")
 		.l2c_aux_val = 0x0,
 		.l2c_aux_mask = ~0x0,
 	MACHINE_END
 
 	mdesc_best = &__mach_desc_GENERIC_DT;
-#endif
 
 	if (!dt_virt || !early_init_dt_verify(dt_virt))
 		return NULL;

arch/arm/mach-dove/Makefile

 # SPDX-License-Identifier: GPL-2.0
-ccflags-$(CONFIG_ARCH_MULTIPLATFORM) := -I$(srctree)/arch/arm/plat-orion/include
+ccflags-y := -I$(srctree)/arch/arm/plat-orion/include
 
 obj-y				+= common.o
 obj-$(CONFIG_DOVE_LEGACY)	+= irq.o mpp.o

arch/arm/mach-mv78xx0/Makefile

 # SPDX-License-Identifier: GPL-2.0
-ccflags-$(CONFIG_ARCH_MULTIPLATFORM) := -I$(srctree)/arch/arm/plat-orion/include
+ccflags-y := -I$(srctree)/arch/arm/plat-orion/include
 
 obj-y				+= common.o mpp.o irq.o pcie.o
 obj-$(CONFIG_MACH_DB78X00_BP)	+= db78x00-bp-setup.o

arch/arm/mach-mvebu/Makefile

 # SPDX-License-Identifier: GPL-2.0
-ccflags-$(CONFIG_ARCH_MULTIPLATFORM) := -I$(srctree)/arch/arm/plat-orion/include
+ccflags-y := -I$(srctree)/arch/arm/plat-orion/include
 
 AFLAGS_coherency_ll.o		:= -Wa,-march=armv7-a
 CFLAGS_pmsu.o			:= -march=armv7-a

arch/arm/mach-orion5x/Makefile

 # SPDX-License-Identifier: GPL-2.0
-ccflags-$(CONFIG_ARCH_MULTIPLATFORM) := -I$(srctree)/arch/arm/plat-orion/include
+ccflags-y := -I$(srctree)/arch/arm/plat-orion/include
 
 obj-y				+= common.o pci.o irq.o mpp.o
 obj-$(CONFIG_MACH_DB88F5281)	+= db88f5281-setup.o