[LibOS+Pal] manifest: Remove support for loader.exec and sgx.sigfile

Supporting these options complicates the design of Graphene and loading
logic significantly, providing little useful functionality:
- loader.exec:
    - the main user of it were our tests
    - worked only for the first process spawned inside Graphene, as it
      was a unidirectional manifest->binary mapping, so the child
      process didn't know about the corresponding manifest.
- sgx.sigfile:
    - probably all existing usages of it were completely redundant
    - was resolved relatively to CWD instead of the executable location,
      which made it mostly useless

From now on, the correct location of the files is:
- either place the manifest and sigfile next to the binary, with a
  matching name, or
- create a symlink to the binary in the folder where manifests are
  stored and launch it through this symlink

Examples/nodejs/.gitignore

@@ -1 +1,2 @@
+/nodejs
 /OUTPUT

Examples/nodejs/Makefile

@@ -21,7 +21,7 @@ GRAPHENEDEBUG = none
 endif
 
 .PHONY: all
-all: nodejs.manifest pal_loader
+all: nodejs.manifest | nodejs pal_loader
 ifeq ($(SGX),1)
 all: nodejs.token
 endif
@@ -63,6 +63,7 @@ nodejs.manifest: nodejs.manifest.template nodejs-trusted-libs
 # Generate SGX-specific manifest, enclave signature, and token for enclave initialization
 nodejs.manifest.sgx: nodejs.manifest helloworld.js
 	$(GRAPHENEDIR)/Pal/src/host/Linux-SGX/signer/pal-sgx-sign \
+		-exec nodejs \
 		-libpal $(GRAPHENEDIR)/Runtime/libpal-Linux-SGX.so \
 		-key $(SGX_SIGNER_KEY) \
 		-manifest $< -output $@
@@ -72,7 +73,9 @@ nodejs.sig: nodejs.manifest.sgx
 nodejs.token: nodejs.sig
 	$(GRAPHENEDIR)/Pal/src/host/Linux-SGX/signer/pal-sgx-get-token -output $@ -sig $<
 
-# Extra executables
+nodejs:
+	ln -s $(NODEJS_DIR)nodejs $@
+
 pal_loader:
 	ln -s $(GRAPHENEDIR)/Runtime/pal_loader $@
 
@@ -84,7 +87,7 @@ check: all
 
 .PHONY: clean
 clean:
-	$(RM) *.manifest *.manifest.sgx *.token *.sig pal_loader OUTPUT
+	$(RM) *.manifest *.manifest.sgx *.token *.sig nodejs pal_loader OUTPUT
 
 .PHONY: distclean
 distclean: clean

Examples/nodejs/nodejs.manifest.template

@@ -2,8 +2,6 @@
 #
 # This manifest was prepared and tested on Ubuntu 18.04.
 
-# Executable to load into Graphene and run.
-loader.exec = file:$(NODEJS_DIR)nodejs
 loader.argv0_override = nodejs
 
 # LibOS layer library of Graphene. There is currently only one implementation,