rtc: initialize emulated RTC from environment variable

Up to now the emulated RTC is initialized using the U-Boot build time.

With this patch the environment variable 'rtc_emul_epoch' can be used to
provide a better initial time. The variable is a decimal string with
the number of seconds since 1970-01-01. Here is an example where the RTC
had not been probed yet:

    => setenv rtc_emul_epoch 1610109000
    => date
    Date: 2021-01-08 (Friday)    Time: 12:30:00

If the variable does not exist, the U-Boot build time is used as fallback.

The environment variable may be set when shutting down the operating system
if the U-Boot environment is exposed to the OS (cf. ENV_IS_IN_FAT and
ENV_IS_IN_EXT4).

Suggested-by: Pablo Sebastián Greco <pgreco@centosproject.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>