cmd_nvedit.c: make error message more helpful

cmd_nvedit.c: make error message more helpful

When calling getenv_f() with a too small buffer, it would print an
error message like this:

	env_buf too small [32]

This is not really helpful as it does not give any indication which of
the calls might have failed.  Change this into:

	env_buf [32 bytes] too small for value of "hwconfig"

so we know at least which variable caused the overflow; this usually
allows to quickly find the related code as well.

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