ARM: boot: Make dd on MacOS happy

Turns out dd on MacOS does not like '1M' as size descriptor, but
wants '1m'. To prevent us breaking Linux builds (as it does not
accept 1m), just use the actual number of bytes explicitely instead.

src/system/boot/platform/u-boot/Jamfile

@@ -151,7 +151,7 @@ actions BuildUImageScript1
 rule BuildUBootSDImage image : files
 {
 	Depends $(image) : $(files) ;
-	SDIMAGE_BLOCK_SIZE on $(image) = 1M ;
+	SDIMAGE_BLOCK_SIZE on $(image) = 1048576 ; # 1M
 	SDIMAGE_SIZE on $(image) = $(HAIKU_BOARD_SDIMAGE_SIZE) ;
 	SDIMAGE_FDISK on $(image) = $(HOST_SFDISK) ;
 	SDIMAGE_FDISK_SCRIPT on $(image) =