repart: always honour `--discard=no`

repart: always honour `--discard=no`

Currently, even if `--discard=no` is passed to `systemd-repart`, the
`context_discard_gap_after()` function still runs normally, discarding
e.g. all blocks between the GPT and the start of the first partition.

This can lead to issues on some embedded devices, where this space
holds the bootloader and shouldn't be modified (creating a protective
partition there is not always possible due to the specifics of the boot
process of some ARM-based SoC's).

This commit ensures passing `--discard=no` would be enough to ensure
the bootloader isn't wiped in such cases.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@gmail.com>