fastboot: mt85xx: add command to flash/erase emmc hwpart

This patch includes the following:
1. Add fastboot command to erase the whole EMMC_USER
2. Add fastboot command to flash image at EMMC_BOOT1
3. Add fastboot command to erase the whole EMMC_BOOT1
4. Enale CONFIG_FASTBOOT_MMC_BOOT1_SUPPORT for mt8518

diff --git a/configs/mt8518_ap1_emmc_defconfig b/configs/mt8518_ap1_emmc_defconfig
index e7c9d63..50219dd 100644
--- a/configs/mt8518_ap1_emmc_defconfig
+++ b/configs/mt8518_ap1_emmc_defconfig
@@ -21,6 +21,7 @@
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x56000000
 CONFIG_FASTBOOT_BUF_SIZE=0x1E00000
+CONFIG_FASTBOOT_MMC_BOOT1_SUPPORT=y
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_DM_MMC=y