environment variables: introduce new helpers

This introduces some new environment variable helpers and updates
the existing ones. Newly introduced are:

getenv_bool: read a bool variable
getenv_ul: read an unsigned long variable
getenev_uint: read an unsigned int variable
getenv_nonempty: like normal getenv, but does return NULL instead of an
                 empty string

All new helpers take a pointer to the value. This value is only modified
when the variable exists. This allows the following programming scheme:

	unsigned int myvalue = sanedefault;

	getenv_uint("myvalue", &myvalue);

So without checking the return value myvalue contains the best possible
value.

getenv_ull is updated to this scheme.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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