mmc_core: Add kconfig to override boot part size

Since punchboot these days actually uses the extcsd to report the
correct size of boot partitions we need a way to be backwards compatible
with existing systems.

This option allows the user to override the boot partition size to
something less than the physical size.

Most users should leave this disabled.

src/drivers/mmc/Kconfig

@@ -24,6 +24,20 @@ config MMC_CORE_HS200_TUNE
         and other conditions that might affect timing to get good tap delay
         value.
 
+config MMC_CORE_OVERRIDE_BOOT_PART_SZ
+    bool "Override boot partition size"
+    default n
+    depends on MMC_CORE
+    help
+        Most user want to set this to 'n'. Under some circumstances it can
+        be useful to ignore what extcsd report and forcibly set the boot partition
+        size to something else.
+
+config MMC_CORE_BOOT_PART_SZ_KiB
+    int "Override boot partition size (KiB)"
+    depends on MMC_CORE_OVERRIDE_BOOT_PART_SZ
+    default 0
+
 config IMX_USDHC
     depends on SOC_FAMILY_IMX
     select MMC_CORE

src/drivers/mmc/mmc_core.c

@@ -648,8 +648,16 @@ static int mmc_setup(void)
     LOG_INFO("Pre EOL %x", mmc_ext_csd[EXT_CSD_PRE_EOL_INFO]);
 */
 
+    lba_t boot_part_last_lba = 0;
+
+#ifdef CONFIG_MMC_CORE_OVERRIDE_BOOT_PART_SZ
+    boot_part_last_lba = CONFIG_MMC_CORE_BOOT_PART_SZ_KiB * 2 - 1;
+#else
+    boot_part_last_lba = mmc_ext_csd[EXT_CSD_BOOT_MULT] * 256 - 1;
+#endif
+
     bio_dev_t d = bio_allocate(0,
-                               mmc_ext_csd[EXT_CSD_BOOT_MULT] * 256 - 1,
+                               boot_part_last_lba,
                                512,
                                mmc_cfg->boot0_uu,
                                "eMMC BOOT0");
@@ -671,7 +679,7 @@ static int mmc_setup(void)
         return rc;
 
     d = bio_allocate(0,
-                       mmc_ext_csd[EXT_CSD_BOOT_MULT] * 256 - 1,
+                       boot_part_last_lba,
                        512,
                        mmc_cfg->boot1_uu,
                        "eMMC BOOT1");