Switch flasher script to subprocess

This avoids any issues re: quoting the commands we execute, such as
fdtput to modify the DTB sent to the flashing process.

The build script already uses subprocess, so this doesn't introduce any
new dependencies overall.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Wrap any env var settings in quotes

This allows the user to pass the following to the flashing script:

    --env bootcmd 'usb start ; dhcp zImage'

rather than having to manually escape the commands for U-Boot:

    --env bootcmd 'usb start \; dhcp zImage'

(The quoting in both cases is for the shell invoking tegra-uboot-flasher,
not for U-Boot's command-line. The removed escaping was to work around
the lack of quoting/escaping when passing the user's command to U-Boot's
setenv command during flashing.)

This change will interact badly with the user wanting to use single
quotes in environment variable values, but hopefully that's less likely,
and can be fixed later if needed.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
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v2: Fix typo in commit description.