update-alternatives.bbclass: run update-alternatives firstly in postinst script

update-alternatives.bbclass: run update-alternatives firstly in postinst script

Recipes like postfix run command newaliases in postinst, but newaliases is
installed as newaliases.postfix, it needs run update-alternatives to update it
to newaliases, so there was an error when installed postinst on target.

Fixed:
$ opkg install postfix
Configuring postfix.
///var/lib/opkg/info/postfix.postinst: line 4: newaliases: command not found

Run update-alternatives firstly will fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>