base: remove platform.mk and syscall.mk

The purpose of those libraries is now covered by the kernel-specific
syscall-<kernel> libraries.

repos/base/lib/mk/startup.inc

@@ -1,6 +1,5 @@
 SRC_S       += crt0.s
 SRC_CC      += _main.cc init_main_thread.cc
 REP_INC_DIR += src/include
-LIBS        += syscall
 
 vpath %.cc $(BASE_DIR)/src/lib/startup

repos/base/lib/mk/syscall.mk

@@ -1,4 +0,0 @@
-#
-# Pseudo syscall library used on platforms that do not need a library for
-# providing syscall bindings.
-#

repos/base/mk/dep_lib.mk

@@ -96,20 +96,6 @@ else
 DEP_VAR_NAME := DEP_$(LIB).lib
 endif
 
-#
-# Add platform preparation dependency
-#
-# We make each leaf library depend on a library called 'platform'. This way,
-# the 'platform' library becomes a prerequisite of all other libraries. The
-# 'platform' library is supposed to take precautions for setting up
-# platform-specific build environments, e.g., preparing kernel API headers.
-#
-ifeq ($(LIBS),)
-ifneq ($(LIB),platform)
-LIBS += platform
-endif
-endif
-
 #
 # Check if the requirements of the target are satisfied
 #