imx: Introduce CONFIG_SPL_FORCE_MMC_BOOT to force MMC boot on falcon mode

This change tries to fix the following problem:

- The board boots (to be more precise - ROM loads SPL) from a slow SPI-NOR
  memory.
  As a result the spl_boot_device() will return SPI-NOR as a boot device
  (which is correct).

- The problem is that in 'falcon boot' the eMMC is used as a boot medium to
  load kernel from its partition.
  Calling spl_boot_device() will break things as it returns SPI-NOR device.

To fix this issue the new CONFIG_SPL_FORCE_MMC_BOOT Kconfig flag is
introduced to handle this special use case. By default it is not defined,
so there is no change in the legacy code flow.

Signed-off-by: Lukasz Majewski <lukma@denx.de>

arch/arm/mach-imx/spl.c

@@ -178,7 +178,18 @@ int g_dnl_bind_fixup(struct usb_device_descriptor *dev, const char *name)
 /* called from spl_mmc to see type of boot mode for storage (RAW or FAT) */
 u32 spl_boot_mode(const u32 boot_device)
 {
+/*
+ * When CONFIG_SPL_FORCE_MMC_BOOT is defined the 'boot_device' is used
+ * unconditionally to decide about device to use for booting.
+ * This is crucial for falcon boot mode, when board boots up (i.e. ROM
+ * loads SPL) from slow SPI-NOR memory and afterwards the SPL's 'falcon' boot
+ * mode is used to load Linux OS from eMMC partition.
+ */
+#ifdef CONFIG_SPL_FORCE_MMC_BOOT
+	switch (boot_device) {
+#else
 	switch (spl_boot_device()) {
+#endif
 	/* for MMC return either RAW or FAT mode */
 	case BOOT_DEVICE_MMC1:
 	case BOOT_DEVICE_MMC2:

common/spl/Kconfig

@@ -608,6 +608,15 @@ config SPL_MMC_SUPPORT
 	  this option to build the drivers in drivers/mmc as part of an SPL
 	  build.
 
+config SPL_FORCE_MMC_BOOT
+	bool "Force SPL booting from MMC"
+	depends on SPL_MMC_SUPPORT
+	default n
+	help
+	  Force SPL to use MMC device for Linux kernel booting even when the
+	  SoC ROM recognized boot medium is not eMMC/SD. This is crucial for
+	  factory or 'falcon mode' booting.
+
 config SPL_MMC_TINY
 	bool "Tiny MMC framework in SPL"
 	depends on SPL_MMC_SUPPORT