fastboot: mmc: extend flash/erase for both emmc hwpart 1 and 2

Update the code and the configs for eMMC boot and userdata
partitions acces
- FASTBOOT_MMC_BOOT_SUPPORT: boot partition 1 and 2 (erase/write)
- FASTBOOT_MMC_BOOT1_NAME: boot partition 1, default name="mmc0boot0"
- FASTBOOT_MMC_BOOT2_NAME: boot partition 2, default name="mmc0boot1"

This patch also removes the unnecessary dependency with
ARCH_MEDIATEK and EFI_PARTITION.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>