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>

target/linux/x86/image/Makefile

@@ -68,6 +68,7 @@ ifneq ($(CONFIG_GRUB_IMAGES),)
 	$(INSTALL_DIR) $(KDIR)/root.grub/boot/grub $(KDIR)/grub2
 	$(CP) $(KDIR)/bzImage $(KDIR)/root.grub/boot/vmlinuz
 	grub-mkimage \
+		-p /boot/grub \
 		-d $(STAGING_DIR_HOST)/lib/grub/i386-pc \
 		-o $(KDIR)/grub2/core.img \
 		-O i386-pc \
@@ -106,6 +107,7 @@ define Image/Build/iso
 	$(INSTALL_DIR) $(KDIR)/root.grub/boot/grub $(KDIR)/grub2
 	$(CP) $(KDIR)/bzImage $(KDIR)/root.grub/boot/vmlinuz
 	grub-mkimage \
+		-p /boot/grub \
 		-d $(STAGING_DIR_HOST)/lib/grub/i386-pc \
 		-o $(KDIR)/grub2/eltorito.img \
 		-O i386-pc \