fixup modules/coreboot: Fix purism-blobs dependency for librem_l1um

Two := assignments were factored out together, the second overwrote the
first.  Fix to +=, and remove the nitrokey assignment since it came
from a branch.

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>

Author: JonathonHall-Purism

modules/coreboot

@@ -96,8 +96,7 @@ modules-y += $(coreboot_module)
 coreboot_dir := $($(coreboot_module)_dir)
 coreboot_base_dir := $($(coreboot_module)_base_dir)
 
-$(coreboot_module)_depends := $(if $(CONFIG_PURISM_BLOBS), purism-blobs)
-$(coreboot_module)_depends := $(if $(CONFIG_NITROKEY_BLOBS), nitrokey-blobs)
+$(coreboot_module)_depends += $(if $(CONFIG_PURISM_BLOBS), purism-blobs)
 
 # coreboot builds are specialized on a per-target basis.
 # The builds are done in a per-target subdirectory