Create clock module

[vinc]

Before:

• src/sys/clock.rs
• src/sys/cmos.rs
• src/sys/time.rs

After:

• src/sys/clk/mod.rs
• src/sys/clk/boot.rs
• src/sys/clk/epoch.rs
• src/sys/clk/rtc.rs
• src/sys/clk/cmos.rs
• src/sys/clk/sync.rs
• src/sys/clk/timer.rs

[vinc]

This last commit is a breaking change for the API and the device files:

• The clock at /dev/clk/boot (previously at /dev/clk/uptime) returns the number of seconds since boot
• The clock at /dev/clk/epoch (previously at /dev/clk/realtime) returns the number of seconds since epoch
• The clock at /dev/clk/rtc (previously at /dev/rtc) returns the value of the RTC

[vinc]

This last commit prevent panicking when writing to some device files where writing was not implemented like copy something /dev/clk/epoch, instead the operation will return an error.