soc/amd/picasso/Makefile.inc: force an error if PSPBTLDR_FILE is not set

Currently, if PSPBTLDR_FILE is empty, the md5sum will hang forever on
stdin, leading to the appearance of a hung script. This is
confusing.

There's no option to md5sum to say "you must use this file", so instead,
use dd with if to ensure we at least get an error if the file is not found.

Not optimal, but better than what we have now.

Change-Id: Ia13035bc592bdf2a515dfd2e052ae9135e218612
Signed-off-by: Ronald G Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43677
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jeremy Soller <jeremy@system76.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/soc/amd/picasso/Makefile.inc b/src/soc/amd/picasso/Makefile.inc
index b3af1fd..aa9d8c9 100644
--- a/src/soc/amd/picasso/Makefile.inc
+++ b/src/soc/amd/picasso/Makefile.inc
@@ -129,7 +129,7 @@
 $(error CONFIG_PSP_BOOTLOADER_FILE was not defined)
 endif
 PSPBTLDR_FILE=$(realpath $(call strip_quotes, $(CONFIG_PSP_BOOTLOADER_FILE)))
-$(info Adding PSP $(shell md5sum $(PSPBTLDR_FILE)))
+$(info Adding PSP $(shell dd if=$(PSPBTLDR_FILE) | md5sum))
 
 # types = 0x8 and 0x12
 PSP_SMUFW1_SUB1_FILE=$(FIRMWARE_LOCATE)/SmuFirmwareRV2.csbin