nerves_heart: remove wildcard install location

While the wildcard part didn't break, the environment variable setting
in the script failed with the Buildroot update. Rather than fix, specify
the install path exactly using the Erlang update to expose the ERTS
version.

NOTES:

1. Target installation hasn't been used in a long time so I deleted it.
   The mix release creation logic copies ERTS over from the staging
   directory.
2. The install command no longer creates parent directories
   automatically. This makes it fail if the ERTS directory path isn't
   right. This is nice since when it didn't fail, you get the default heart
   program that doesn't pet the hardware watchdog and then the device
   reboots.

package/nerves_heart/nerves_heart.mk

@@ -10,19 +10,14 @@ NERVES_HEART_LICENSE = MIT
 NERVES_HEART_LICENSE_FILES = LICENSE
 NERVES_HEART_DEPENDENCIES = erlang
 NERVES_HEART_INSTALL_STAGING = YES
+NERVES_HEART_INSTALL_TARGET = NO
 
 define NERVES_HEART_BUILD_CMDS
 	$(MAKE1) $(TARGET_CONFIGURE_OPTS) -C $(@D)
 endef
 
-define NERVES_HEART_INSTALL_TARGET_CMDS
-	ERTS_BIN=$(ls $(TARGET_DIR)/usr/lib/erlang/erts-*/bin);\
-	if -e "$(ERTS_BIN)"; then $(INSTALL) -D -m 755 $(@D)/heart $(ERTS_BIN)/heart; fi
-endef
-
 define NERVES_HEART_INSTALL_STAGING_CMDS
-	ERTS_BIN=$(ls $(STAGING_DIR)/usr/lib/erlang/erts-*/bin);\
-	if -e "$(ERTS_BIN)"; then $(INSTALL) -D -m 755 $(@D)/heart $(ERTS_BIN)/heart; fi
+	$(INSTALL) -m 755 $(@D)/heart "$(STAGING_DIR)/usr/lib/erlang/erts-$(ERLANG_ERTS_VSN)/bin/heart"
 endef
 
 $(eval $(generic-package))