misc: fs_loader: Add request_firmware_into_buf_via_script() for flexible firmware...

misc: fs_loader: Add request_firmware_into_buf_via_script() for flexible firmware loading via U-Boot script

This commit introduces a new API,
request_firmware_into_buf_via_script(), to the fs_loader framework.
This function allows firmware to be loaded into memory using
a user-defined U-Boot script, providing greater flexibility for
firmware loading scenarios that cannot be handled by static file
paths or device/partition selection alone.

Key features:
- The API runs a specified U-Boot script (by name), which is responsible
  for loading the firmware into memory by any means (e.g., load from MMC, USB, network, etc.).
- The script must set two environment variables: 'fw_addr'
  (the memory address where the firmware is loaded) and
  'fw_size' (the size of the firmware in bytes).
- The function validates these variables, copies the firmware into a newly
  allocated buffer (using memdup), and returns the pointer
  via the provided double pointer argument.
- The maximum allowed firmware size is checked to prevent buffer overflows.
- The environment variables are cleared after use to avoid stale data.
- Detailed error messages are provided for all failure conditions to aid debugging.

Usage example:
1. Define a U-Boot script in the environment that loads the firmware
   and sets the required variables:
   => env set my_fw_script 'load mmc 0:1 ${loadaddr} firmware.bin &&
   env set fw_addr ${loadaddr} && env set fw_size ${filesize}'

2. In your code, call the new API:
   void *fw_buf = NULL;
   int ret = request_firmware_into_buf_via_script(&fw_buf, 0x46000000, "my_fw_script");
   if (ret < 0)
		return ret;

This approach allows board integrators and users to customize the firmware
loading process without modifying the source code,
simply by changing the script in the U-Boot environment.

Signed-off-by: Lucien.Jheng <lucienzx159@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[trini: Fix printf of size_t needing to use %zx]
Signed-off-by: Tom Rini <trini@konsulko.com>