usb: gadget: fastboot: Add fastboot erase

Adds the fastboot erase functionality, to erase a partition
specified by name. The erase is performed based on erase group size,
to avoid erasing other partitions. The start address and the size
is aligned to the erase group size for this.

Currently only supports erasing from eMMC.

Signed-off-by: Dileep Katta <dileep.katta@linaro.org>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>

include/fb_mmc.h

@@ -6,3 +6,4 @@
 
 void fb_mmc_flash_write(const char *cmd, void *download_buffer,
 			unsigned int download_bytes, char *response);
+void fb_mmc_erase(const char *cmd, char *response);