Add hash table support as base for new environment code

This implementation is based on code from uClibc-0.9.30.3 but was
modified and extended for use within U-Boot.

Major modifications and extensions:

* hsearch() [modified / extended]:
  - While the standard version does not make any assumptions about
    the type of the stored data objects at all, this implementation
    works with NUL terminated strings only.
  - Instead of storing just pointers to the original objects, we
    create local copies so the caller does not need to care about the
    data any more.
  - The standard implementation does not provide a way to update an
    existing entry.  This version will create a new entry or update an
    existing one when both "action == ENTER" and "item.data != NULL".
  - hsearch_r(): Instead of returning 1 on success, we return the
    index into the internal hash table, which is also guaranteed to be
    positive.  This allows us direct access to the found hash table
    slot for example for functions like hdelete().
* hdelete() [added]:
  - The standard implementation of hsearch(3) does not provide any way
    to delete any entries from the hash table.  We extend the code to
    do that.
* hexport() [added]:
  - Export the data stored in the hash table in linearized form:
    Entries are exported as "name=value" strings, separated by an
    arbitrary (non-NUL, of course) separator character. This allows to
    use this function both when formatting the U-Boot environment for
    external storage (using '\0' as separator), but also when using it
    for the "printenv" command to print all variables, simply by using
    as '\n" as separator. This can also be used for new features like
    exporting the environment data as text file, including the option
    for later re-import.
  - The entries in the result list will be sorted by ascending key
    values.
* himport() [added]:
  - Import linearized data into hash table.  This is the inverse
    function to hexport(): it takes a linear list of "name=value"
    pairs and creates hash table entries from it.
  - Entries without "value", i. e. consisting of only "name" or
    "name=", will cause this entry to be deleted from the hash table.
  - The "flag" argument can be used to control the behaviour: when
    the H_NOCLEAR bit is set, then an existing hash table will kept,
    i. e. new data will be added to an existing hash table;
    otherwise, old data will be discarded and a new hash table will
    be created.
  - The separator character for the "name=value" pairs can be
    selected, so we both support importing from externally stored
    environment data (separated by NUL characters) and from plain text
    files (entries separated by newline characters).
  - To allow for nicely formatted text input, leading white space
    (sequences of SPACE and TAB chars) is ignored, and entries
    starting (after removal of any leading white space) with a '#'
    character are considered comments and ignored.
  - NOTE: this means that a variable name cannot start with a '#'
    character.
  - When using a non-NUL separator character, backslash is used as
    escape character in the value part, allowing for example fo
    multi-line values.
  - In theory, arbitrary separator characters can be used, but only
    '\0' and '\n' have really been tested.

Signed-off-by: Wolfgang Denk <wd@denx.de>

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