split some rtc funcs out of hwclock and into an rtc header/lib so that the new rtcwake applet as well as hwclock can utilize the same code
diff --git a/libbb/Kbuild b/libbb/Kbuild
index c4aac95..2fb1b24 100644
--- a/libbb/Kbuild
+++ b/libbb/Kbuild
@@ -115,6 +115,8 @@
lib-$(CONFIG_DF) += find_mount_point.o
lib-$(CONFIG_MKFS_MINIX) += find_mount_point.o
lib-$(CONFIG_SELINUX) += selinux_common.o
+lib-$(CONFIG_HWCLOCK) += rtc.o
+lib-$(CONFIG_RTCWAKE) += rtc.o
# We shouldn't build xregcomp.c if we don't need it - this ensures we don't
# require regex.h to be in the include dir even if we don't need it thereby