eco32: entry: documentation update

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: remove commented out lines in Makefile
eco32: debug cruft
eco32: move PT_ORIG_GPR2 store to ret_from_exception

This way, -1 will be written to orig_gpr2 when a sigreturn jumps
here. Also, it removes one unnecessary store in the syscall path.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>