boot/grub2: update _SITE to use BR2_GNU_MIRROR
The grub2 package is distributed on the GNU project servers. See [1]
and [2]. Buildroot has the BR2_GNU_MIRROR configuration which can
be used for that purpose. See [3].
For consistency with all other GNU packages, this commit updates
the _SITE to use BR2_GNU_MIRROR.
Note: Commit [4] introduced the grub2 package using BR2_GNU_MIRROR.
Commit [5] changed it to use "http://ftp.gnu.org/gnu/grub" without
providing a justification for that change.
[1] https://www.gnu.org/software/grub/grub-download.html
[2] https://www.gnu.org/prep/ftp.html
[3] https://gitlab.com/buildroot.org/buildroot/-/blob/2024.08.2/Config.in#L286
[4] https://gitlab.com/buildroot.org/buildroot/-/commit/c24fdb3680019d18f0cb7c02a90975ee9fe9613c
[5] https://gitlab.com/buildroot.org/buildroot/-/commit/5ffafd2353f5d67dc74c7f3784dfe48d4080435e
Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
(cherry picked from commit 51858c4a34f73c52ac9ff36ba7facf8a8ab9b711)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Julien Olivain