build: convert build system to CMake

Makefile

@@ -41,6 +41,13 @@ LIBC_SRC_DIRS := stdio string ctype
 LIBC_C_FILES := $(foreach dir,$(LIBC_SRC_DIRS),$(shell find libc/$(dir) -type f -name *.c))
 LIBC_OBJ := $(addprefix $(BUILD_DIR)/,$(LIBC_C_FILES:.c=.o))
 
+####################################
+# Userboot source files
+####################################
+
+USERBOOT_C_FILES := $(shell find userboot -type f -name *.c)
+USERBOOT_OBJ := $(addprefix $(BUILD_DIR)/,$(USERBOOT_C_FILES:.c=.o))
+
 BUILD_ID := $(shell tools/socks.buildid --arch $(SOCKS_ARCH))
 
 CWARNINGS := -Wall -Werror -pedantic -Wno-language-extension-token -Wno-unused-function  -Wno-gnu-statement-expression