lib/hashtable.c: add CONFIG_ENV_MIN_ENTRIES

This patch adds a new config parameter for adjusting the calculation of
hash table size when importing a buffer.

When importing a extremely small buffer (e.g. the default_environment)
the old calculation generated a hash table which could hold at most the
buffer content but no more entires.

The new calculation add a fixed number of entries to the result to fit
better for small import buffers. This amount may be configured by the
user in board file to adjust the behaviour.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>