lib: move rtc-lib.c to lib

Function rtc_to_tm() is needed for FAT file system support even if we don't
have a real time clock. So move it from drivers/ to lib/.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

lib/Makefile

@@ -135,6 +135,7 @@ obj-$(CONFIG_SSCANF) += sscanf.o
 endif
 
 obj-y += date.o
+obj-y += rtc-lib.o
 obj-$(CONFIG_LIB_ELF) += elf.o
 
 #