[BMC] Align is_bmc_supported check for the new infra (#385)

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#### What I did
The BMC support check in `generate_dump` was aligned with the new infrastructure flow
(sonic-net/sonic-buildimage#26544).
Instead of checking for platform-local `bmc.json` assumptions, the logic now validates host-side role and runtime BMC data from the canonical source used by the system.

#### How I did it
- Updated `generate_dump` BMC detection to use `device_info.is_switch_host()` together with
 `device_info.get_bmc_data()`.
- Removed reliance on legacy platform-folder `bmc.json` existence behavior for this path.
- Kept the `generate_dump` flow behavior intact while aligning detection to runtime
 `/etc/sonic/bmc.json`-based APIs.

#### How to verify it
- Run `show techsupport` (or `generate_dump`) on:
 - a switch-host platform with valid BMC data and verify BMC-related flow executes.
 - a non-switch-host / missing-BMC-data platform and verify BMC path is skipped.

Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)

Author: mssonicbld

scripts/generate_dump

@@ -2271,13 +2271,7 @@ save_log_files() {
 #  0 if BMC is supported, 1 otherwise
 ###############################################################################
 is_bmc_supported() {
-    local platform=$(python3 -c "from sonic_py_common import device_info; print(device_info.get_platform())")
-    # Check if the required file exists
-    if [ ! -f /usr/share/sonic/device/$platform/bmc.json ]; then
-        return 1
-    else
-        return 0
-    fi
+    python3 -c "from sonic_py_common import device_info; import sys; sys.exit(0 if (device_info.is_switch_host() and device_info.get_bmc_data()) else 1)"
 }
 
 ###############################################################################