at91bootstrap: fix overlap linker issue

The linker script of the at91bootstrap package has to be modified when
built from gcc-4.6.x version. Indeed a section named text.startup is
created and has to be added into the text section.

Signed-off-by: Gregory Hermant <gregory.hermant@calao-systems.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>