common, env: optimize boottime

when creating the hashtable, for each environmentvariable
getenv(ENV_CALLBACK_VAR) and getenv(ENV_FLAGS_VAR) is called,
which costs at this point a lot of time. So call this two
getenv() calls only once.

Boottime on the ids8313 board without this patch:

2013-12-19 13:38:22,894:  NAND:  128 MiB
2013-12-19 13:38:27,659:  In:    serial
(~4.8 sec)

Bootime with this patch on the ids8313 board:

2013-12-19 13:40:25,332:  NAND:  128 MiB
2013-12-19 13:40:25,546:  In:    serial
(~0.2 sec)

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Wolfgang Denk <wd@denx.de>