gummiboot: bump to 28

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gummiboot: exlibify

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gummiboot: add scm

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>