[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/python-simple/.gitignore

@@ -1 +1,2 @@
 *.pyc
+/python

Examples/python-simple/Makefile

@@ -23,7 +23,7 @@ GRAPHENEDEBUG = none
 endif
 
 .PHONY: all
-all: python.manifest pal_loader
+all: python.manifest pal_loader | python
 ifeq ($(SGX),1)
 all: python.manifest.sgx python.token python.sig
 endif
@@ -110,6 +110,7 @@ python.manifest: python.manifest.template python-trusted-libs python-trusted-scr
 
 python.manifest.sgx: python.manifest
 	$(GRAPHENEDIR)/Pal/src/host/Linux-SGX/signer/pal-sgx-sign \
+		-exec python \
 		-libpal $(GRAPHENEDIR)/Runtime/libpal-Linux-SGX.so \
 		-key $(SGX_SIGNER_KEY) \
 		-manifest $< -output $@
@@ -119,7 +120,9 @@ python.sig: python.manifest.sgx
 python.token: python.sig
 	$(GRAPHENEDIR)/Pal/src/host/Linux-SGX/signer/pal-sgx-get-token -output $@ -sig $<
 
-# Extra executables
+python:
+	ln -s $(PYTHONEXEC) $@
+
 pal_loader:
 	ln -s $(GRAPHENEDIR)/Runtime/pal_loader $@
 
@@ -133,7 +136,7 @@ check: all
 
 .PHONY: clean
 clean:
-	$(RM) *.manifest *.manifest.sgx *.token *.sig pal_loader OUTPUT* *.PID
+	$(RM) *.manifest *.manifest.sgx *.token *.sig python pal_loader OUTPUT* *.PID
 	$(RM) -r scripts/__pycache__
 
 .PHONY: distclean

Examples/python-simple/python.manifest.template

@@ -2,15 +2,6 @@
 #
 # This manifest was prepared and tested on Ubuntu 16.04/18.04 and tested with
 # Python 3.5 and 3.6.
-#
-# Python must be run with the pal_loader:
-#
-# ./pal_loader python.manifest <script>
-
-# The executable to load in Graphene. By default, PYTHONHOME points to the
-# system installation. To run Python from a local installation, specify PYTHONHOME
-# when running `make` in this directory.
-loader.exec = file:$(PYTHONEXEC)
 
 # Graphene environment, including the path of the library OS and the debug
 # option (inline/none).