pmaports.cfg: add supported_fastboot_depends

Add this new variable so pmbootstrap can use it to determine which
depends need to be installed for the fastboot flashing method. It used
to be android-tools,avbtool but now it's only android-tools as avbtool
became part of android-tools.

Related: issue 1779
Related: https://postmarketos.org/pmaports.cfg

pmaports.cfg

@@ -9,3 +9,4 @@ supported_base_nofde=True
 supported_install_boot_label=pmOS_i_boot
 supported_mkinitfs_without_flavors=True
 supported_arches=x86_64,x86,aarch64,armv7,armhf
+supported_fastboot_depends=android-tools