boot/opensbi: fix build with empty OPENSBI_PLAT

Fix the following build failure when OPENSBI_PLAT is empty raised since
commit 9b5b7165deb91bad3cdb1ea0dd8f940eff75f3a9:

/usr/bin/install -m 0644 -D /nvmedata/autobuild/instance-4/output-1/build/opensbi-0.9/build/platform//firmware/fw_jump.bin /nvmedata/autobuild/instance-4/output-1/images/fw_jump.bin
/usr/bin/install: cannot stat '/nvmedata/autobuild/instance-4/output-1/build/opensbi-0.9/build/platform//firmware/fw_jump.bin': No such file or directory

Fixes:
 - http://autobuild.buildroot.org/results/8033327f090e4a3d84a7fce1f62b14fdf89dbd89

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit bd061466f90e3f17d47d4e1641590d60c07a6cac)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/opensbi/opensbi.mk

@@ -70,6 +70,7 @@ OPENSBI_INSTALL_IMAGES = YES
 OPENSBI_FW_IMAGES += payload
 endif
 
+ifneq ($(OPENSBI_PLAT),)
 define OPENSBI_INSTALL_IMAGES_CMDS
 	$(foreach f,$(OPENSBI_FW_IMAGES),\
 		$(INSTALL) -m 0644 -D $(@D)/build/platform/$(OPENSBI_PLAT)/firmware/fw_$(f).bin \
@@ -78,6 +79,7 @@ define OPENSBI_INSTALL_IMAGES_CMDS
 			$(BINARIES_DIR)/fw_$(f).elf
 	)
 endef
+endif
 
 # libsbi.a is not a library meant to be linked in user-space code, but
 # with bare metal code, which is why we don't install it in