Support running run_flashy_remote.sh in release build
Summary:
When we release a prebuilt flashy to OSS, we need to point to the right flashy binary, since we are no longer building flashy from the project root.
Add a new flag `path-to-flashy` that allows pointing to the release path of flashy.
In the github release, make sure we have the right folder structure
```
. flashy
|---scripts
|--- run_flashy.sh
|--- run_flashy_remote.sh
```
Test Plan:
# Running in release folder, path to flashy provided
```
release/scripts $ ./run_flashy_remote.sh --device mtd:flash0 --host fboss9310996-oob.snc1 --dry-run --imagepath ../flash-wedge40 --path-to-flashy ../flashy
Running in dry-run mode
Running a remote upgrade on 'fboss9310996-oob.snc1' with image '/data/users/linhaolee/openbmc/tools/flashy/release/flash-wedge40'
Continue (y/n)?y
path-to-flashy provided: /data/users/linhaolee/openbmc/tools/flashy/release/flashy
Making installation directories on OpenBMC...
Copying flashy...
Copying image...
Copying upgrade script...
Finished dry run
```
# Running in scripts folder, path to flashy not provided
```
openbmc/tools/flashy/scripts $ ./run_flashy_remote.sh --device mtd:flash0 --host fboss9310996-oob.snc1 --dry-run --imagepath ../release/flash-wedge40
Running in dry-run mode
Running a remote upgrade on 'fboss9310996-oob.snc1' with image '/data/users/linhaolee/openbmc/tools/flashy/release/flash-wedge40'
Continue (y/n)?y
path-to-flashy not provided, building flashy...
Making installation directories on OpenBMC...
Copying flashy...
Copying image...
Copying upgrade script...
Finished dry run
```
Reviewed By: deathowl
fbshipit-source-id: 27bcbaf8606864c2b768455ff2a9ee15e2dbfaa3
