Makefile: at91bootstrap: allow to run project rules.
Makefile: at91bootstrap: allow to run project rules.

Internal project rules are now accessible using:
$ make at91bootstrap_<rule>

Example:
$ make at91bootstrap_menuconfig

Signed-off-by: Gaël PORTAY <1666f60cfa201a60b0ea694168a0b6932673170a@gmail.com>

gazoo74/at91nandflash