fsgen: Use prebuilt_{root,any} in case install_in_root is set

If any of the other modules relative dir matches (eg lib/) it
would hit a neverallow as installing in the root directory
is allowlisted.

Test: m --no-skip-soong-tests
Test: m on tegra device with
PRODUCT_COPY_FILES += \
    tegra21x_xusb_firmware:recovery/root/tegra21x_xusb_firmware \

Change-Id: I157826703974544614f0049b8fa157358e7b17a7