scripts: improved size validator for updater image

[hedger]

What's new

• scripts: update.py: reduced reservation size for flash memory; improved error messages; added checks for updater images

Verification

• In targets/f7/src/main.c, add unused globals:

#ifdef FURI_RAM_EXEC
uint8_t dummy_pad[20 * 1024] = {1};
#else
uint8_t dummy_pad[114 * 1024] = {1};
#endif

int main(void) {
    dummy_pad[1] += 1;

• Build update bundle
• Try 104 kb pad for flash to check other error message

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[github-actions]

Compiled f7 firmware for commit 60554e4e:

• 📦 Update package
• 📥 DFU file
• ☁️ Web/App updater
• 📊 Size report