Add new "bootpart enable" command to enable boot partitions

This adds a new option to enable the boot from eMMC
by writing to the 179 ext_csd register.

The example below shows as to enable the mmcblk0boot0
partition and also set/reset the ACK bit).

$ ./mmc bootpart enable 1 1 /dev/mmcblk0boot0
$ ./mmc extcsd read /dev/mmcblk0boot0 | grep PARTITION_CON
Boot configuration bytes [PARTITION_CONFIG: 0x49]

Reported-by: Youssef TRIKI <youssef.triki@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/mmc.c b/mmc.c
index c27fc24..6927b70 100644
--- a/mmc.c
+++ b/mmc.c
@@ -65,6 +65,11 @@
 		"Set the eMMC writeprotect status of <device>.",
 	  NULL
 	},
+	{ do_write_boot_en, -3,
+	  "bootpart enable", "<boot_partition> " "<send_ack> " "<device>\n"
+		"Enable the boot partition for the <device>.\nTo receive acknowledgment of boot from the card set <send_ack>\nto 1, else set it to 0.",
+	  NULL
+	},
 	{ 0, 0, 0, 0 }
 };