target/linux/x86/image: add explicit prefix to grub-mkimage command

In the latest version of grub-mkimage, the prefix option is mandatory.
Not supplying it fails with:
```
Prefix not specified (use the -p option).
```

In grub-2.02-beta2 a DEFAULT_DIRECTORY was defined
in `include/grub/osdep/hostfile_unix.h` as:
```
 #if defined (__NetBSD__)
 /* NetBSD uses /boot for its boot block.  */
 # define DEFAULT_DIRECTORY  "/"GRUB_DIR_NAME
 #else
 # define DEFAULT_DIRECTORY  "/"GRUB_BOOT_DIR_NAME"/"GRUB_DIR_NAME
 #endif
```

Where:
* GRUB_BOOT_DIR_NAME == boot
* GRUB_DIR_NAME == grub

This was used if the -p option was omitted.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>