mx28: Fix elftosb source link in README.mx28_common

mx28: Fix elftosb source link in README.mx28_common

The documented link to elftosb package tarball is not accessible,
change to another working link.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Detlev Zundel <dzu@denx.de>
Acked-by: Marek Vasut <marex@denx.de>