sunxi: Create helper function veryfing valid boot signature on MMC

This patch extracts checking for valid SD card "eGON.BT0" signature from
`board_mmc_init` into function `sunxi_mmc_has_egon_boot_signature`.

Buffer for mmc sector is allocated and freed at runtime. `panic` is
triggered on malloc failure.

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>
CC: Roy Spliet <r.spliet@ultimaker.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
[hdegoede@redhat.com: Small bugfix to make it work for devs other then mmc0]
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
diff --git a/arch/arm/include/asm/arch-sunxi/mmc.h b/arch/arm/include/asm/arch-sunxi/mmc.h
index cb52e64..3da360b 100644
--- a/arch/arm/include/asm/arch-sunxi/mmc.h
+++ b/arch/arm/include/asm/arch-sunxi/mmc.h
@@ -127,4 +127,5 @@
 #define SUNXI_MMC_COMMON_RESET			(1 << 18)
 
 struct mmc *sunxi_mmc_init(int sdc_no);
+int sunxi_mmc_has_egon_boot_signature(struct mmc *mmc);
 #endif /* _SUNXI_MMC_H */