[Blackfin] arch: declare default INSTALL_PATH for Blackfin ports

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>

diff --git a/arch/blackfin/Makefile b/arch/blackfin/Makefile
index 0edc402..fe254f8 100644
--- a/arch/blackfin/Makefile
+++ b/arch/blackfin/Makefile
@@ -119,6 +119,7 @@
 	$(Q)$(MAKE) $(clean)=$(boot)
 
 
+INSTALL_PATH ?= /tftpboot
 boot := arch/$(ARCH)/boot
 BOOT_TARGETS = vmImage
 PHONY += $(BOOT_TARGETS) install