README.md: fix markdown link formatting

Consistently use reference-style markdown links in the README. This
won't show the URL next to the linked text for the [Github] and [U-Boot]
links and link the text "Bootlin toolchain repository" instead of
showing it verbatim with the URL next to it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>