Call hwmatch only on the grub-pc platform

Call hwmatch only on the grub-pc platform

Call hwmatch only on i386/pc as it is only available there.
This avoids "error: can't find command `hwmatch'." on e.g., x86_64/efi.

The equivalent behavior is linux_gfx_mode=keep because grub is special:
the `if hwmatch` clause is true on that error and `$match = 0` is true
too, as it is undefined (confirmed in grub shell.) A quick fix for now.

Before and After:

    grub> hwmatch
    error: can't find command `hwmatch'.

    grub> echo $grub_platform
    efi

    grub> echo $linux_gfx_mode
    keep

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Bug-Ubuntu: https://bugs.launchpad.net/bugs/1840560
Bug-Debian: https://bugs.debian.org/990836
Forwarded: no
Last-Update: 2020-08-20

Gbp-Pq: Name hwmatch-only-on-grub-pc-platform.patch